Physics for You [updated] 0198375719, 9780198375715

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D

1og

totql momentum ofter collison (moss

(0.01

0.4

x

x

velocity)ox".

After

+0.39) x10 10

400 m/s

: i x moss x speed2 (poge 109) : I x 0.01 x 4002 = 800J Totol KE o/ter collision -- ) x moss x speedz : , x 0.4 x 702 :20J Totol KEbeJore

Betore

collision

-_-_---> 10 m/s

The differen ce,780 J, is tronsferred to heot energy ond sound energy during the collision.

Note: momentum is conserued in q collision, but kinetic energy is not t37

More exomples of momentum in collisions In ony collision, o force is exerted for o length of time. The exomples on poge 136 showed thot lhe longer the time of o collision, the smaller the Jorce exerted. This ideo is used to design sofety into cqrs. The front ond bock of o cor ore designed to crumple, in order to spreod out the time of o collision, ond so reduce the force on you. (See poge 69.) An oir bog olso spreods out the time. A seot-belt is designed to stretch slightly, to spreod out the time of the crosh even further, ond so reduce the force on you to o sofe level : A motor-cyclist's sqfety helmet is podded inside so os to extend the time of ony collision. In on opposite woy, o hqmmer is designed from hord metol. This is so thot the time of the collision is os smoll os possible, ond so the force of the hommer on o noil is os lorge os possible.

Explosions An explosion is the opposite of o collision objects move oport insteod of coming together. A rocket uses o controlled explosion. The rocket moves one woy while the hot goses move the opposite woy. (See olso poge 85.) The goin of momentum of the rocket is equol ond opposite to the momentum of the hot goses thot ore ejected: A gun firing o bullet is like o rocket ejecting fuel. The bullet goins momentum in one direction, while the gun recoils with momentum in the opposite direction. moss

4kg

ls

0.01 kg

+ 400 m/s

o vector, let the the right.

explosion

ofter

xv) 1m/s

138

Summory Momentum (kg

m/s)

:

mqss x velocity. It is o vector quontity.

(kg)

(m/s)

Newtorfs second

Low:

So:

x time

Force

(Compore: Force

x

Force

chongg in r.ngmentum tlme toKen

-

)

Questions

1.

Copy ond complete:

109)

totol momentum before : totol momentum dter collision (or explosion) collision (or explosion)

q) Momentum:....x b) Itiso....quontity. c) Itsunitis.... d) Newton's Second Low con be written

Force:mossx ...or Force : . . . . of chonge of . . .

4. A truck of moss 2 kg trovels ot 8 m/s towords o stotionory truck of moss 6 kg. After colliding, the trucks link ond move off together.

o) Whot b) Whot

os

Momentum stotes: . . . . is conserved in ony collision but

c) Exploin the opporent loss in energy.

5.

In o seo bottle, o connonboll of moss 30 kg wos fired of 200 m/s from o connon of moss 3000 kg. Whot wos the recoilvelocity of the gun?

6.

A heovy cor A, of moss 2000 kg, trovelling ot 10 m/s, hos o heod-on collision with o sports cor B, of moss 500 kg. If both cors stop deod on colliding, whot wos the velocity of B?

7.

A mon weoring o bullet-proof vest stonds still on roller skqtes. The totol moss is 80 kg. A bullet of moss 20 groms is fired ot 400 m/s. It is stopped by the vest ond folls to the ground. Whot is then the velocity of the mon? How does this compore with whot you see in TV films?

8.

A Soturn V Moon rocket burns fuel ot the rote of 13 000 kg in eoch second. The exhoust goses rush out of 2500 m/s. o) Whot is the chonge in momentum of the fuel in eoch second? b) Whot is the thrust? (See olso question 16, poge 133.)

kinetic....isnot.

A moss of 3 kg is moving with o velocity of 2mls. Whot is its momentum?

3. o) Exploin the Physics of this limerick: A dashing young footballer, Paul, Scored a goalwith a one-kilo ball, But a similar kick To a one-kilo brick Atlade Paul bawl, and then fall, and then crawl.

b) When you jump down from

o toble, why your you legs rother thon keep do bend them rigid? Why do porotroopers rollon londing? Why would you prefer to foll on to o bed thon on to concrete? c) Why ore seot-belts designed to stretch in o collision? Why is the front of o cor designed to collopse in o serious collision? d) Whv does o cricket botsmon, o tennis ployer or o golf ployer 'follow through'? e) Professor Messer invents o foom-rubber hommer ('to moke less noise'). Will it work? Why?

is their common velocity? is the KE before ond ofter the

collision?

.

e) The Principle of Conservotion of

2.

rote of chonge of momentum

= chonge in momentum : mv - mat distance : chongZ in KE: *^u' -f,muz frompqge

Principle of Conservotion of Momentum: (if no externolforce is octing)

f)

:

Further questions on poge 141. 139

Further questions on mechonics

1.

Some sfudents corry out on experiment to find out how o spring stretched when smoll loods were odded to it. The results ore shown in the toble. One reoding is incorrect.

Load (N)

0

2

4

6

I

10

12

14

Extension (mm)

0

16

32

58

64

80

96

112

o) b)

6.

@ffii

tyre front view A tyre compony stoted thot the oreo of the tyre in contoct with the rood wos obout the some os the oreo of the sole of one of your shoes. o) Describe (with o diogrom) how you would estimote the oreo of sole of your shoe. t3] b) The cor weighs 12 000 N. Whot is the force octing on one tyre if the weight of the cor is evenly dishibuted omongst the tyres? [1] c) If the oreq of contoct of the pre is 80 cmz (0.008 m2), colculote the pressure of the qir in the tyre. 121 side view of

Use these results to plot o groph. [3 marks] Use your groph to find: i) the extension when the lood is 3 N ii) the lood for on extension of 40 mm. [2]

c) Lobel the incorrect point on the groph with the letter

2.

'E'.

t1l

A person pushes q door open by opplying o force of 10 N ot o point 0.8 m from the hinge of the door. The turning effect or moment of this force, in N m, is: A 0.08 B 8.0 C 10.8 D 72.5 E 80.0

7

large

piston

area =

area =

2 cm2

A uniform metre rule of moss 100 g bolonces ot the 40-cm mork when o moss X is ploced ot the 10-cm mork. Whot is the volue of X?

100 cm2

oil

The diogrom obove shows the principle of the hydroulic cor jock. o) If o force of 50 N is opplied to the smoll piston, colculote the pressure produced in the oil qt t2l b) Whot is the pressure exerted by the oil ot

4. The diogrom shows o simple mqchine for lifting woter from o river. +1

F

N

small piston

(Nt)

3.

The diogrom shows side ond front views of o cor tyre in contoct with the rood.

X.

.5

m+

c) weight of bucket ot water = 200 N

Y?

tll

Colculote the upword force, F, octing on the lorge t1l

piston.

I

o) b)

Colculote the turning force (moment) of the bucket of l2l Colculote the downwords force the operotor must use to bolonce the moment of the bucket of [4] (AOA)

woter.

woter.

5.

A sponner wos used to undo o nut. A force of 20 newtons (N) wos opplied ot o distqnce of 20 centimehes (cm) from the nut. Colculote the moment of the force being used to undo the nut. [3] (AOA)

140

x

8.

A skydiver is folling from on oeroplone. o) Nome i) force X, ii) force Y. t2) b) Stote how eoch force chonges os the sky diver speeds up. 12) c) Why does the skydiver reoch o steody speed (terminol velocity)? 12) d) Describe ond exploin whot hoppens when the sky diver opens the porochute.

Note: although a question may be marked (AQA) etc, it will often be relevant to olher examinations as well

[3]

(wJEC)

Further questions on mechonics

9.

Suson is riding her motorbike ot 100 km/h oround o bend, which is port of o circle. o) Although her speed stoys the some she is occeleroting. Exploin l2l b) Drow o diogrom to show the horizontol forces on Suson ond the motorbike. 12)

13.

why.

1O. The toble gives doto obout o folling skydiver. Speed (m/s) 0

Weight (N)

Air resistance (N) Resultant force (N)

700

0

700

10

280

420

20

490

210

30

630

70

40

14. A golfer strikes q stotionory

golf boll with his golf club. The moss of the boll is 0.045 kg. o) Whot is the momentum of the bollbefore

0

700

o) Copy ond complete the tqble. 12) b) Use the doto to drow o groph of resultont force ogoinst

speed.

b)

t3l

c) Whot is meont by terminal t:elocity? l1l d) Stqte the terminol velocity of this skydiver.

11.

t1l The toble shows the onnuol risk of being in o rood occident where somebody is injured.

15.

150

UK USA

385 675

0.150 0.385 0.675

0.009 0,026 0.023

roqds?

l2l

Use the ideo of momentum to exploin why 'crumple zones' should reduce possenger

crosh.

t3l

(A0A)

16. o) Choose on exomple of o renewoble energy source ond suggest whot you would soy to persuode people to support the development of this energy source. [2] b) Mony people might disogree with your ideos. Suggest whot orguments they might use. 12) (oCR)

o) Which of these countries oppeors to hove the sofest

it is struck?

When the club strikes the boll it is in contqct for 0.001 s ond exerts q force of 3600 N on the boll. Colculqte the velocity ot which the boll leoves the club. [3] (AoA)

injury in o cor

Country Annual risk per Annual risk per Annual risk per person (7o) car (o/ol 100,000 people lreland

A trolley hoving o mqss of 2kg ond velocity 4 m/s collides with onother holley hoving o moss of 1 kg which is ot rest. If the trolleys stick together ofter the impoct, cqlculote: q) the totol momentum before the collision, b) the common velocity ofter the collision, c) the kinetic energy before the collision, d) the kinetic energy ofter the collision. Why ore the onswers to (c) qnd (d) different?

tll

b) Exploin why ollthe figures in the first column ore lorger thon oll the figures in the second column. t1] c) The risk per cor in the USA ond the UK is obout the some but the risk per person is much higher for the USA. Suggest reosons for this. 12) d) Despite the risks, people still use the roods. Suggest reosons for this. l2l

17. o) Give one reoson

why solor power would not be o suitoble replocement for power stotions burning fossilfuels. t1l b) Exploin why some people believe thot nucleor power would not be on occeptoble replocement. [3] (wJEC)

12. Andrew is designing o lift to corry

people in qn office block. He knows thot the tension in the coble depends on o the weight of the lift ond the people in it, forces from the lift motor ond the brokes. 'Hetheuses o computer to colculote the coble tension in hundreds of different situotions. o) Exploin why it is importont for Andrew to know the coble tension. 12) b) Exploin why Andrew uses q computer insteod of doing lots of experiments. l2l

18.

A villoge uses solor ponels qnd windmills os

olternotive enerEg sources. o) The ponels ond windmills ore expensive to instoll. Eventuolly ollthe money spent on them willbe recovered. Exploin why. l2l b) The energy of the oir hitting the blodes of o windmill is 20 000J eoch second. The energy honsferred to the power lines is 5000 J eoch second. Colculote the efficiency of the windmill. l2l (OCR) 747

Further questions on mechonics

22. 19. o) For eoch of these 4 opplionces stote the moin energy hqnsfers which toke ploce in them: Battery Lamp

b)

Loudspeaker Microphone

(iii)..

iv) c)

o) b)

A cool-fired power stotion generotes elechicol power for the notionol grid. Exploin how the energy from the coql is used to generote elechicity. [3] (WJEC)

21. o) i) ii)

c)

tll

Exploin why cor designers ore concerned with fueleconomy when designing q new cor. [3] (Eoex)

Stote two environmentolproblems coused by burning cool to generote

electuicity.

23.

H#[:"Jt]uous

used

Energy content of coal Total quantity of coal used per

day

2.66 x 1010 J per tonne .18 289 tonnes

doy stotion.

trqnsfers.

t42

[4]

(AOA)

The diogrom shows o coql-fired power stotion.

[[[[

D

electrical energy outout of zab [,t.J

T-1 -

Exploin where the energy in cool originolly come from. l2l b) Cqlculote how efficient the power stotion is of chonging the energy in the cool into elechicolenergy. t3l c) In whot form is most energy lost from this power stotion? t1l d) Whot eventuolly hoppens to this 'lost'

5oo MW

i) the totol electricol enerEg output per doy [3] ii) the totol input of cool energy per 12) iii) the efficiency of the power I3l c) Energy is conserved. Exploin whot hoppens to ollwosted energy during energy [2] (AOA)

con.

o)

4

Use the given doto to colculqte:

. J of energy 'wasted' by car engine

movement.

fuel giving an energy output of 1000 MJ

l2l

generators Power

(ii)

Whot ore the missing volues (i), (ii), (iii)? [3] Colculote how efficient the cor engine is ot tronsferring the energy from petrol into useful t2l Two students ore discussing the diogrom. The first soys thot none of the energy releqsed from the crude oil is reolly lost. The other soys thot oll of the energy releosed from the crude oil is reolly lost. Whot do you think? Exploin your onswer os fully os you

How moy these environmentol problems be reduced? t2) b) Some doto for Didcot coql-fired power stotion ore given below. Number of

move

i oil

(i) . . J of energy used to refine oil and transport petrol to filling station

tll

fuel.

t I

dtfr

Describe how cors con be designed to reduce the omount of petrol used when trovelling ot o steody speed. t2) The primory source of energy for most cors is crude oilwhich is o fossil fuel. i) Give one other exomple of o fossil

ii) 20.

(

.J

of energy used to make car

IU ,

l4l

When o driver opplies the brokes of o cor, its kinetic enerEg is honsferred to heot energy. i) Where is this heqt energy produced? [1] ii) Why does frequent speeding up ond slowing down increose the omount of petuol used by o cor? t3l iii) Exploin why petrolis used up even when o cor is trqvelling of q constont

speed.

The diogrom shows whot hoppens to eqch 100 joules of energy from crude oil when it is used os pehol in o cqr.

energg?

24.

[1]

(AOA)

Energy connot be creoted ond it connot be deshoyed, but energy con be wosted. Discuss this [7] (AOA)

stotement.

25.

Give three odvontoges of o hydro-elecfuic power stotion over o cool-fired power

stotion.

[3]

(AOA)

Further questions on mechonics

29. ln o downhill roce o mon

26.

skis q totol distonce of 3000 m. The verticol height the mon drops is 800 m.

Aworker on o building site roises o bucket full of cement ot o slow steody speed, using o pulley os shown:

SOOO

rn

8OO m

F

o) The mon ond his ski equipment hove q totql moss of 90 kg. Whot is their

weight?

b) Copy the diogrom ond drow ond lobel

The weight of the bucket ond cement is 200N. The force F exerted by the worker is 210N.

o) Why is F bigger thon the weight

cement?

rope?

c) How

c)

slope.

t1l

d)

much work is done on the bucket ond cement? t3] d) Whot kind of energy is goined by the bucket? t1l e) How much work is done by the worker?[2] f) Where does the energy used by the worker come from? [1] (AoA)

27.

A cor is trovelling ot 30 m/s on o level roqd. At this speed the cor hos to overcome o totol force opposing motion of 600 N. o) How for does the cor go in 10 seconds? [1] b) How much work does the cor engine do in this time? l2l c) Whot is the power? l2l d) Burning the pekolsupplies 60kI (60 000 J) of energy to the engine every second. Why is your onswer to (c) smoller thon this volue? l2l e) Whot is the efficiency? [2] (oCR)

2E. A cor is trovelling ot o steody speed of 15 m/s. o) Colculote the distonce moved in 10 s. tll b) The totolresisting force (friction ond oir resistonce) is 800 N.

c)

Colculote the work done by the cor in 10 s in overcoming this resisting force. [4] Colculote the power developed ot the driving [4] (AOA)

wheels.

three forces octing on the mon os he skis down the t3l The qveroge frictionqlforce pushing ogoinst the mon is 210 N. Colculote the totol work done ogoinst friction os he skis down the l2l Cqlculote the loss in grovitotionol potentiol energy of the skier during the t3l Colculote the kinetic energy of the skier os he posses the finish. (lgnore ony work done by the skier t1l Cqlculqte the speed of the skier ot the

slope.

of the

bucket ond I1l The bucket is roised through o height of 4 m. b) Through whot distonce does the worker

pullthe

l2l

e)

f)

rqce.

himself.)

finish.

[3]

(ocR)

30.

A motor is used on o building site to roise o block of stone. The weight of the block is 720N qnd it is roised 20min24 s. Colculote: o) the work done on the block, t3] b) the useful power supplied by the motor. [3]

31.

A boy of moss 50 kg roces up o flight of 40 steps, eoch of height 15 cm, in 5 s. Colculote: o) the work done t3l b) his qveroge power. t3l c) Why is the energy he trqnsfers greoter thon the colculqted work done? [2] (WJEC)

32.

An electric pump whose efficiency is 60% roises woter to o height of 15 m. If wqter is delivered ot the rote of 360 kg per minute, whot is the power roting of the pump?[S] Whot is the energy lost by the pump? t1l WJEC)

743

Further questions on mechonics

35. 33.

The velocity, u, of o cor vqries with time, t, occording to the following toble: t (s)

0

5

10

1

5 20

25 30 35 40

v (m/s)

0

5

1

0

1

5

1

o) b)

c) d) e)

34.

1

5

5

1

5

11

45

50

7 .5 3.5

0

Plot o groph of u ogoinst t. Describe the motion of the cor during eoch of the following periods of time:

t3l

0-15s 15-30s 30-50s.

t3l

Colculote the occelerotion of the cor during the period 0-15 s. How for did the cor hovel in the following periods of time?

t3l

overtoke the mon?

0-15s 15-30s 0-30s

d) Whot wos the dog's time for the roce? e) Use the equotion u : f to colculote the

forces

f)

equolto?

t4]

0

I

12

15

Speed (km/h)

0

48

64

80

g)

An underground troin tokes 1 minute to hqvelfrom one stotion to the next. The troin qccelerqtes from rest to o speed of 25 mA in 20 s, trqvels of this constont speed for 30 s before coming to rest under o uniform broking force. o) Drow o groph to represent the motion. [3] b) From the groph, cqlculote: i) the time thot the trqin is trovelling with o speed greoter thon 20 m/s, ii) the distonce trovelled by the troin when it is moving qt constont speed, iii) the retordotion of the troin, iv) the broking force opplied to stop the hoin, given thot the moss of the troin is 100 000 kg. [6] (wJEC)

37.

A roller-cooster corrioge is releosed ot the top of o steep slope. Then it * occelerotes to the bottom of the slope * is slowed by o woter splosh u continues of constont speed * is stopped by brokes ot the end of the ride. o) Sketch o groph to show how the speed of the roller-cooster cqrrioge chonges during this time. 14) b) Describe the energy chonges involved. t4l

speed.

with

speed.

t1l

Some time loter the thinking time of the driver wos meqsured os 1.0 second. i) Colculote the thinking distonce for this driver when driving ot 9 t1l ii) A speed of 9 m/s is the some os szkrnlh. Use your groph to find the thinking distonce ot 32 km/h for o driver who hos not hod q tll iii) Whot hos been the effect of the drink on the thinking distonce of the

m/s.

drink.

driver?

[1]

(AOA)

overoge speed of the mon. After 8 seconds, is the speed of the mon increosing, decreosing, stoying the some? Whot is the dog's speed ofter 18 s? (AOA)

36.

On groph poper, drow o grqph of thinking distonce ogoinst t3l Describe how thinking distonce chonges

c) A driver dronk two pints of loger.

7M

overtoke the mon?

[7]

Stote whot forces would be octing on the cqr when it is fuovelling ot constont velocity. Whot is the resultont of these

Thinking distance (m)

b)

o) Whot wos the distonce for the roce? b) After how mony seconds did the dog

c) How fqr from the stort did the dog

When o cor driver hos to reoct ond opply the brokes quicklg the cor trovels some distonce before stopping. Pqrt of this distonce is cqlled the 'thinking distonce'. This is how for the cor trovels while the driver reocts to o dongerous sifuotion. The toble below shows the thinking distonce (m) for vorious speeds (km/h).

o)

A mon runs o roce ogoinst o dog. Here is o groph showing how they moved:

Further questions on mechonics

40.

38.

A cor of moss 900 kg is trovelling ot 30 m/s. A smoll child runs in front. The driver's reoction time is 0.6 s qnd it tokes o further 2 s to broke the cor to o hqlt. Colculote: o) the 'thinking distonce' trovelled by the cor during the reoction time, 12] b) the broking decelerotion, [3] c) the overoge broking force, t3] d) the chonge in kinetic enerEg, t3l e) the work done by the brokes. t1l f) Drow o speed :time groph for the cqr. t2l g) Exploin how the groph will chonge if : i) the driver drinks qlcohol, ii) the roqd is wet. 12)

39.

o

400

A front-wheel drive cor is hovelling ot constont velocity. The forces octing on the cor ore shown in the diogrom obove. Q is the force of the oir on the moving cor. P is the totql upword force on both front wheels. o) Exploin why i) P = 4000 N, ii) 12) b) Colculote the moss of the t1l c) The 400 N driving force to the left is suddenly doubled. i) Colculote the resultont force driving the cor forword. ii) Colculote the occelerotion of the cor. iii) Drow o sketch groph showing how the velocity of the cor chonges with time. (Stort your groph just before the driving force is doubled.) tsl d) i) Possengers in o cor ore qdvised to weor o sofety belt. Exploin, in terms of Newton's Lows, how o sofety belt con reduce injuries. 12) ii) Whot other design feofure in q cor con offer protection in o crqsh? [4] (WJEC)

Q=400N. cor.

o E

=o o q)

o

.:ti..;..

q)

0

o.2

0

.2

o.4

1.4

time (s)

o)

From the groph colculote i) the time thot he took to reoch the woter, ii) the height of the diving boord, iii) Ion s decelerotion in the woter, iv) the retording force on lon in the woter, v) the depth in the woter thot Ion

reqched.

b) i) How for did

[13]

Ion trovel in the first 0.5

s

of the dive? ii) Whot is the loss of Ion s potentiol enerEg qfter he hos follen this distonce? iii) Use the groph to colculote Ions kinetic enerEg 0.5 s ofter he hos stqrted to dive. iv) Give on explonotion for the difference between the onswers to port (ii) ond

(iii).

[10]

(AOA)

N

10 000 N

Ion hos o moss of 70 kg. He dives from o high diving boord. His verticolvelocity ot different times is shown in the groph below. Grovitotionol field strength is 10 N,&g.

E

N P

41.

A cor is trovelling ot 20mls. The driver brokes constontly until the cor stops. This tokes2 s, once the brokes come on. o) Colculote the decelerotion of the cor. t3l b) The cor ond driver hove o moss of 1000 kg. Colculote the broking force on the cor. [3] (oCR)

42. A moss of 5 kg chonges velocip steodily from 4mls to 24mls in 10s. Colculote: o) the overoge velocity, b) the distonce trovelled in 10 s, c) the occelerotion, d) the force needed for this occelerotion, e) the momentum ofter 10 s, f) the kinetic energy ot the stort, g) the kinetic enerEg qfter 2 s.

t3]

t3l [3] [3] t3]

t3l I3l

t45

chapter 2O

Our beoutiful plonet Eorth is o sphericol spoceship held in orbit round the Sun by the pull of grovity. The otmosphere ond everything on Eorth is held to it by grovity. This grovitotionql force olso crushes the Eorth itself so thot the rocks inside ore very dense. Inside the Eqrth there ore 3 moin loyers, qs shown: - The core is very hot metol, mostly iron ond nickel. Most of it is liquid, but the centre is under such high pressure thqt it is solid. The Eorth's mognetic field is probobly due to currents in the core. The core is hot becouse it is rodiooctive (see poge 338). - The montle is mostly solid rock but some of it is 'slushy' - o mixture of solid ond hot molten (liquid) rock, rother like lovo from o volcono. The montle con move very slowly in giont convection currents.

-

core very hot, very dense, metal liquid except at centre

mantle hot, less dense, rock, mostly solid

crust thin, less dense, rock

The crust is o very thin solid loyer, like the shell on on egg.

solid

convection currents

How do we know this? We moke deductions by observing the effects of. earthquakes. Whenever there is on eorthquoke, shock woves colled seismic woves trovelinto the Eqrth. There ore 3 kinds of seismic wove: P, S ond L. They trovel through the Eorth in different wqys.

The L woves ore woves thqt trovel olong the Eorth's crust. These ore the woves thot domoge buildings: The P (primory) woves: o These ore longitudinal woves, like sound woves. Think of them os 'push-ond-pull' woves thot you con moke on o 'slinky' spring (see olso poge 166, bottom): o They trovel quickly (30 times os fqst os sound in oir). o They con trovelthrough liquids os well os solid rock. The S (secondory) woves: o These ore trqnsuerse woves. Think of them qs 'shokesidewoys' woves, like the tronsverse wove you con moke on q rope or o 'slinky' spring (see poge 166, top) o They trovelmore slowly thon the P-woves. o They con trqvel through solid rock but not through liquids, becouse they ore tronsverse woves. 146

.< >

---->

P-wave

--->

S-wave

earthquake

earthquake

Trovelling through the Eorth The P-woves ond the S-woves both trovel through the Eorth, but they behove differently: A

Becouse the core is liquid, the S-woves connot trovelthrough it. This creotes o shadow-zone'.

..s

/

Seismogroph stotions oll over the world detect the woves. In the diogroms, stotions A, B, C ond D ollreceive S-woves os well os P-woves. But stotions X ond Y only receive P-woves. From this we know thot the core is liquid, ond its size con be colculoted.

D

D

/////

/

C

X

Y

shadow zone

S-waves

F-waves epicentre

The 2 diogroms con be combined into one:

Notice thot the poths ore curved. This hoppens becouse of refraction. Both the woves trovel foster if the rock is denser. The deeper they go into the Eorth, the denser the rock ond so they speed up. Becouse they ore chonging speed, the woves are refracted (see poge 168 ond poge 184). If the density of the rock chonges groduolly, the woves ore bent in curves. If the density chonges suddenly, the woves ore bent shorply.

A

D P and S waves

c

B

# P-waves only shadow

shadow zone

zone

X

Example

An eorthquoke occurs 9000 km from seismic stqtion D (see diogrom obove) The aueragespeedsfor PondSwoves qre P: 10km/s S:6km/s Whot is the gop between the times thot the 2 woves orrive ot D? From poge 122

rime token

'. Time token by P-woves

Time token by S-woves

distonce trovelled speed a simple seismometer

=

ffi

=eoos

:

9000 km

:1500s

6

krVt

station

D

noise

intervol = 1500s-900s =

600s:

10min

t 'P'

In proctice this colculotion is done in reverse, to find the distonce to the eorthquoke (or nucleor bomb test)

arrive

Selsmic waves

t

'S'arrive

t 'L'waves

time

ona

747

(not to scale)

The Eorth moves round the Sun, in on orbit, which is (olmost) o circle. The Eorth is pulled into this orbit by o centripetal Jorce (see poge 70). This force is the grovitotionol force between the moss of the Sun ond the moss of the Eorth.

gravity\, l

lorbit=1year

revolution

= 'l day

Experiment 20.1 A day Use o white tennis bqll os the Eorth. Sketch the moin continents on it, ond mork the position of your school. Insert o knitting needle os its north-south oxis. In o dorkened room, use o lomp for the Sun. Turn your 'Eorth' slowly, so your 'school' is in doylight ond then night. For the reol Eorth, one revolution tokes 24 hours, o complete doy. For other plonets it is different. The Sun 'rises' in the eost. Which woy should you rotote your 'Eorth'?

Experiment 20.2 A year Wolk with your 'Eorth' in o complete circle round the 'Sun'. Mork out the 72 months of the yeor. For the reol Eorth, 1 complete orbit tokes 365] doys (this is why we need o leop yeor every 4 yeors). Experiment 20.3 Polar and equatorial regions Look corefully of the surfoce of your 'Eorth' in the light from the 'Surf At the equotor, where the Eorth's surfoce is focing the Sun, it looks bright. But neor the poles the surfoce is curved owoy ond so the energy is spreod out more. Why is the Eorth cold ot the poles?

231/z'

Sun's rays

'+

The seqsons of the yeor In foct, the Eorth's oxis is not ot right-ongles to the orbit os you might hove ossumed. It is tilted ot on ongle of 23)". Experiment

20.4

Summer and winter

Repeot experiment 2O.2bulwith the oxis oluroys

tilted towards the same end oI the classroom. Look corefully of how much of the Sun's energy orrives of the northern hemisphere of different times of the yeor. This exploins three things: o When the northern hemisphere is focing owoy from the Sun, it is winter. Six months loter (holf on orbit) it is focing towords the Sun (in summer) Whot hoppens in the southern hemisphere? o The Sun oppeors higher in the sky in summer. It hos o higher eleuation obove the horizon. o Doylight losts for more hours in summer. Imogine you ore ot the North Pole. At whot time of the yeor is it o) doylight for 24 hours? b) night-time for 24 hours?

1€

March (not to scale)

June

The Moon is o stork ond hostile world. Its moss is smoller thon the Eorth's, so its grovitotionol field is less. It is only one-sixth of the Eorth's field (g = 1.6 kgA\, see poge 131). How high could you jump on the Moon? Becouse its grovity is less, oll the molecules of goses hove escoped, so it hos no otmosphere.

The Moon is o sotellite in circulor orbit round the Eqrth, tropped by grovity. One orbit of the Moon tokes 1 month (1 'moonth'). It orbits the Eorth once every 27 doys.lt qlso spins on its oxis once every 27 doys. This is why it olwoys keeps the some side towords us. It shines by reflecting the light from the Sun The Moon looks different to us ot different times of the month. It hos phoses. The diogrom shows the Moon ot 8 different positions round the Eorth with the 8 phoses Experiment 20.5 Phoses ot' the Moon Use o lomp or qn overheod projector for the Sun's roys, with o white boll for the Moon. Then stond in the centre (os the Eorth) ond look of the 'Moon' os it is moved round in orbit.

An astronaut on the h/loon

+ Sun's rays

^7 \-r---+.(not to scale)

6

Or

Gr.---'--

4

Phases of the Moon 2

Experiment 20.6 Phases of the Moon Observe the reol Moon for o full month, weother permitting. Keep o diory of its oppeoronce eoch evening, o) in writing ond b) in sketches.

G

3456

7

new crescent first gibbous lull gibbous last quarter quarter moon moon

8

crescent

Try to find out when the next lunqr eclipse will occur (see the diogroms on poge 173).

Tides Just os the Eorth pulls on the Moon to keep it in orbit, so the Moon pulls on the Eorth (Newton's Third Low, see poge 84). This grovitotionol force olso pulls on the Eorth's seos so thot the woter piles up. For complex reosons, there ore tu;o bulges os shown: As the Eorth rototes, eoch port of the world gets two high tides eoch doy, ond two low tides. The energy stored in these tides con be used

+ Moon

sea

gravitational pull

(not to scale)

to produce electricity (see poge 15). 149

Our solor system consists of o centrol stor (the Sun), surrounded by plonets, including Eorth. The first six plonets ore visible to the noked eye, ond hove been known since oncient times. The others were discovered through telescopes. The plonets ore held in their orbits by the

grovitotionol pull of the Sun (see poge 71). The plonets vory enormously in their size (see below, with the Sun in yellow on the some scole). They olso vory in their distonce from the Sun (see the diogrom on the opposite poge).This meons thot the conditions vory from plonet to plonet:

I Mercury

.

Mercury is closest to the Sun, ond smoll for o plonet (obout the size of our Moon). It hos no otmosphere ond is covered

2 Venus

Saturn, photographed by the Voyager-2 space probe

o

3

Eorth

e

The side focing the Sun is very hot, obout 430 "C (thot is hot enough to melt leod).

Venus is olmost os big os the Eorth, but very unpleosont. It is covered in clouds of sulphuric ocid, with on otmosphere of corbon dioxide ot very high pressure. Becouse of the CO2 ond the Greenhouse Effect (see poge 48), it is even hotter thon Mercury.

From spoce, Eqrth is o blue plonet with swirls of cloud. It is the only plonet with woter ond oxygen ond living things. It is ot the right distonce from the Sun, with the right chemicols, to support life. Other stors moy hove plonets with the some conditions.

4

5

6

in croters.

Mqrs

a

Jupiter

- the red plonet - is o cold desert of red rocks, with huge mountoins ond conyons. There is no life on Mors. It hqs o thin otmosphere of corbon dioxide, ond two smoll moons. Between Mors ond Jupiter there ore thousonds of rocks, colled osteroids (see the opposite poge).

giont of the plonets. It hos no solid surfoce, being moinly liquid hydrogen ond helium, surrounded by goses ond clouds. The Greot Red Spot is o giont storm, three times lhe size of Eorth. Some of the 16 moons hove volconoes.

7

8

Mors

Uronus

Uronus is onother giont plonet. It looks pole green, with very foint rings ond 15 moons. It wos discovered by Williom Herschel in 1781. It is unusuol becouse its oxis is tilted right over, so thot it is 'lying on its side' os it goes round the Sun.

Jupiter is the cold

Neptune

Neph-rne is the t'win of

Uronus. They ore both obout 4 times the size of Eorth.

Neptune is bluish, due to o thick otmosphere of cold methone. It hos 8 moons, one with volconoes. The Greot Dork Spot is o storm obout the size of Eorth.

The Sun 150

13 \f/

Soturn

Sofurn is onother 'gos giont', very like Jupiter. The beoutiful rings ore not solid. They ore mode of billions of tiny chunks of ice, held in orbit by the pull of Soturn's grovity. As well os the rings, Sofurn hos more thon 20 moons.

Pluto

o

Pluto used to be colled o plonet but now it is colled o dwolf plonet. It wos discovered in 1930. We don't know much obout it. Pluto's orbit is not os circulor os the others. Most of the time its orbit is outside Neptune's, but betrueen 1979 ond 1999 its orbit wos inside Neptune's. Astronomers hor.,e been seorching for o 'plonet 10' beyond Pluto.

Mercury Sun

Venus Ma rs ,.:

Itt

,: ,

,-,

: ''.:;l ,,.,.1,:,,,,

t

.-l::.' : .: .:,,,

.

:

..i:

asteroids

Jupitel

Neptune

Uranus

Saturn

)

l

The distances to the planets, drawn to scale. On this scale, the nearesl star would be 1 km away !

you are here

Experiment 20.7 The solar system Moke o scole model of the solor system: o) For the Sun use o gropefruit or cordboord disc

c) Hold your 'Eorth' of distonce of 12 metres from the 'Sun'. From column B below, hold the other plonets of the correct scole distonces (eg. Jupiter : 5 x Eorth : 5 x L2 : 60m). You'll need to go on the ploying field! On this scole, the neorest stqr would be onother gropefruit obout 3000 km owoy!

of diometer 11cm. b) For the Eorth, moke o boll of plosticine 1 mm in diometer. From column C in the toble below, moke oll the other plonets to the some scole (eg. Jupiter : 11 x Eorth = 11mm).

The moving plonets The orbit of eoch plonet is not quite o circle. It is o slightly-squoshed circle colled on ellipse. They ore not oll trovelling ot the some speed. The plonets neorer the Sun trovel more quickly. They hove o shorter 'yeor' (column H below). Plonets do not give out their own light. They just reflect the Sun's light. Becouse they ore much neorer thon stors, they oppeor to move slowly ogoinst the bockground of the stors:

Two photos of Pluto, taken exactly one day apart

It has moved against the constellation o/ slars.

Here is o toble of doto obout the solor system. Look down eoch column to see whot potterns you con find (see olso question 8, on poge 162)

AB Average distance

c

D

E

F

G

H

I

Diameter

Density

Average temperature

Mass

Surlace

Time for

Number

(Earth = 1)

(kg/m3)

cc)

lrom the Sun The planets

1 Mercury 2 Venus

4

Mars

(millions of

km)

(relative to Earth = 1)

(Earth

=

58

0.4

o.4

5500

+430 to -180

0.1

108

0.7

0.95

5200

+470

0.8

228

1.5

778

5

0.5

1)

gravity

I

(N/ks)

(years)

4

I

5500

10

4000

4

-30

0.1

orbit

ot moons

0.2

0

0.6

0

,1.0:.. 1.9

1

2

Asteroids

5 Jupiter

11

1300

-

150

320

26

12

16

700

-

180

95

11

30

20 + rings

-210

'15

11

84

15

17

12

165

I

0.6

244

1

6 Saturn 7 Uranug 8 Neptune

2870 4497

30

4

1

700

-220

9

5900

39

o,2

2000

-230

Pluto

1427

9.s '19

9 4

1

300

0.002

+ rings

151

Most ostronomers believe thot oll the plonets were formed ot the some time os the Sun obout 4500 million yeors ogo. It begon with o huge cloud of dust, ice ond gos, colled o nebulo. The gos wos mostly hydrogen, with some helium. These goses moke up most of the Universe. Becouse of its own grovity, the cloud begon to fu it got smoller ond smoller, this 'protostor' got hotter ond hotter. (As the porticles fell inwords, they lost potentiol energy but goined kinetic enerEg, so the gos got hotter.)

shrink.

Eventuolly the centre of the cloud wos white hot ond nuclear fusion begon (see poge 156). It become the stor we collthe Sun.

Meonwhile the plonets begon to form. The heot of the Sun pushed some of the lighter gos ond ice into o doughnut-shoped cloud, leoving the heovier dust in the gop. Becouse of grovity the dust begon to stick together, eventuolly moking the 4 rocky inner plonets. In the some woy, the outer plonets formed from the ice, ond then collected the gos to become 'gos gionts' like Jupiter.

Asteroids ore some of the rocky left-overs. Most of them orbit between Mors ond Jupiter. Those thot might collide with the Eorth ore colled Neor Eorth Objects (NEOs).

Meteors ('shooting stors') ore tiny frogments thot burn up in the Eorth's otmosphere. Comets moy be left-overs from when the solor system wos formed. They ore lumps of ice,

dust ond gos. They hove very ellipticol orbits, which bring them close to the Sun ond then for out in the solor system (often beyond Pluto):

When they foll neor to the Sun they speed up os the pull of grovity increoses. The dust ond gos ore blown owoy from the Sun ond shine in the sunlight, to form o long toil. 1s2

The famous Halley's comet, which returns every 76 years

I I I

I

,\ / z\ .'I I

I I

\

o ,t,I

ll ,l

\/ orbits of planets

orbit ol a comet

Imogine firing o gun from the top of o very high mountoin. The shellwould fqll bock to the Eorth, just like o footboll. See A in the diogrom: If the shell is fired foster, it folls forther owoy (see B ond C). If the shell could be fired even foster (ot 8 km/s, 25 times the speed of sound) then it would still foll towords Eorth, but becouse of the curving of the Eorth, the shell would stoy the some height obove the ground (see D):

It is then o satellite, in orbit. Grovity keeps pulling it into orbit (see olso p. 71)

\ l

I

\

I

I

I

I

\

I

\

.-----/

it is still lalling, but does not hit the ground

-/

Sqtellite orbits To stoy in orbit ot o certoin height, the sqtellite must hove the right speed to bolonce the pull of

grovity. Low sotellites hove to trovel foster to stoy up. A sotellite in high orbit tokes longer to complete 1 orbit, just like the outer plonets round the Sun. A sotellite moves very fost, but it con seem to be stonding stilMf the sotellite is put ot just the right height ond speed, it will toke 24 hours to go round the Eorth. This is the some time os the Eorth tokes to spin once, so the sotellite oppeors to stoy over one ploce ! See olso poge 154.

An ostronout in orbitJeels weightless, becouse the ostronout ond his clothes and the spoceship ore oll folling of the sqme rote together. However to become truly weightless the ostronout would hove to trovel for owoy from the grovitotionol force of the Eorth ond the Sun.

Bruce McCandless in 'werglhtless' orbit near his spacecraft He is usrng small jets of gas to manoeuvre.

Grovitotionol force Experiments show thot: There is o grovitotionol pull between oll mosses. Lorger mosses give o stronger pull. The further the distonce owoy, the smoller the grovitotionol pull. In foct, ot twice the distonce the pull is |. At 3 times the distonce, the pull is ]. This is colled on 'inverse squore' Iow. Whot do you think the pullwould be ot 4 times the distonce?

o . o

Distances:

I

.......s Force:

1

2

.{ 1 4

3

-c 1

I

153

Sotellites ore objects thot orbit round the Eorth. They ore lounched by rockets (see poge 160). Like the Moon, they ore held in orbit by the grovitotionol pull of the Eorth (see poge 153). To stoy in o porticulor orbit, they hove to trovel ot the right speed. The higher the sotellite, the slower the speed ond the longer it tokes for 1 complete orbit (see

Launching the space shuttle

the exomple in the box below). I

Sotellites con be lounched into polar orbits or equatorial orbits:

Polor orbits ore usuolly lower. As the Eorth turns on its oxis, the sotellite posses over o different port of the Eorth on eoch orbit. This is useful for spy sotellites ond observotion

:,

ir

equalorra orbit

sotellites (see opposite poge).

lndian Ocean

Equotoriol orbits ore better for some uses. Although o sotellite is moving very fost, it con oppeor to be stotionory. If it is ploced in the right orbit, ot o height of 36 000 km obove the equotor, it tokes 24 hours to go round its orbit. This is the some period os the Eorth, so the sotellite oppeors to hover over one ploce. This is o geo-stationary or sgnchronous orbit. This is ideol for communicotions sotellites.

Pacific Ocean

Atlantic Ocean

The whole Earth can be covered by th ree geo- stati o n ary sate//iles

j ust

Colculoting the speed of o sotellite If the rodius of the orbit is B, then the circumference of the circle is 2

r

E.

If the time for 1 orbit (the 'period') is T. distonce trovelled 2 nR Speed : time token T Example A sotellite ot o height of 700 km obove the Eorth's surfoce, orbits with q period of 100 minutes. Whot is its speed? The rodius of the Eorth is 6400 km. Rqdius of orbit, R: 6400 + 700 km : 7100 km : 7 100 000 m Time of orbit, T : 100 minutes : 100 x 60 seconds : 6000 s Speed

. t54

2nR 2xnx7100000 : T 6000

7435mls (obout 16000mph!)

Con you colculote the speed of o sotellite in o geo-stotionory orbit?

R

-->.

Communicotions sotellites like'lntelsot'

use

o geo-stotionory orbit (see opposite poge). If you telephone to Americo, o microwove rodio signol is tronsmitted from o'dish' qeriol up to the sotellite. The sotellite then tronsmits it down to onother dish oeriol in Americo. See the diogrom on poge 211. Microwoves ore used becouse they trovel in o norrow beom, in o stroight line, ond poss eosily through the Eorth's otmosphere.

An' I ntelsat' communications sate//ite

In the some woy, o TV sotellite ploced in o geo-stotionory orbit over the equotor, con broodcost progrqmmes to oll of Europe:

TV satellite

The woves spreod out (by diffroction, see poge 169) os they leove the dish. You need o dish qeriql on your house to pick up the weok signol (see poge 182).

TV signal

Weother sqtellites like 'Meteosot' ore often put in geo-stotionory orbits (see photo below).

Eorth observotion sotellites like 'Londsot' con give us o detoiled picture of the Eorth's surfoce. They con worn of forest fires ond locust sworms, show up diseosed crops, hurricones ond oil leoks (see below).

transmitter dish

u0[u NEE TV studio

Militory sotellites ore used for spying. Novigotion sotellites ore used by ships, cors,

Astronomicol sotellites con toke shorper

plones to locote their position very occurotely. They use o GPS (Globol Positioning System) to telltheir position on Eorth within 10 metres!

photogrophs outside the Eqrth's otmosphere, see poge 275. They con olso trovel to other plonets.

From a weather satellite. A clear day except snow and cloud in Scotland.

An oil-slick in the Gulf (detected by infra-red, and given aftificial colou)

A Landsat image: a volcano (Vesuvius). Artificial colour (blue:houses in Naples)

155

The Sun is o stor like the others thot we see ot night. Compored to other stors, the Sun is very ordinory ond overoge. The Sun is not burning like o fire - it is o huge controlled hydrogen bomb. The core is ot o temperoture of 74 million "C. ond ot o very high pressure. The hydrogen otoms ore broken into pieces, which smosh into eoch other ot high speed. This con moke the pieces of hydrogen otoms join together ond become helium. Eoch time this hoppens, some moss is losf. It reoppeors os energg (see poge 348).

This is nucleor fusion Hydrogen

nucleor fusion

Helium

*';IJr(i

The Sun is using up its moss ot the rote of 4 million tonnes eoch second ! It hos been doing this for 4500 million yeors, but is still only holf-woy through its life-time. It is o middle-oged stor.

An ultra-violet photograph of the Sun, showing a huge prominence curvrng in the Sun's magnetic field

Twinkle twrnkle sunrty star, /Vow we know lust what you are. Nuclear fusron. burnrnq briqlttly, Nlce of you to swrtcl't off rtrglttly !

The huge omount of energy is rodioted out from the core. Some of this energy eventuolly orrives on Eorth, keeping us olive (poge 101). Inside the Sun there is o constont bottle of forces: the pull of grovity is trying to crush the Sun, while the force of rodiotion from the very hot core is trying to moke it expond. As long os these forces ore balanced, the stor is stoble.

radiation pressure

Eventuolly it will run out of fuel ond chonge to onother kind of stor (see opposite poge).

Sun-spots ore slightly cooler oreos on the Sun's surfoce - lhey ore only of 4000 'C Sun-spots come ond go, being common every 11 yeors (the 'sun-spot' cycle). It is possible thot the Sun's power moy vory slightly from time to time, offecting the Eorth's weother - perhops even cousing the Ice Ages.

currents

photosphere

!

In the night sky, the other stors oppeor to form potterns, colled constellotions. Some fomous ones ore Cossiopeio ond the Plough:

756

The north sky in Winter

Stors eventuolly use up their fuel ond stort to chonge. Whot hoppens then depends on the moss of the stor.

1

Smqll stors like our Sun (yellow dworfs)

After obout 10 000 million yeors, the hydrogen is oll used up ond the stor storts to use helium os o fuel. This mokes heovier elements (up to element 26, iron). While doing this it exponds ond cools to become o red giont. When this hoppens to the Sun, it will swollow up Mercury ond Venus, ond kill oll life on Eorth Loter, the stor uses up oll its fuel ond collopses under grovity to form o very hot ond dense white dworf. Its density is o million times the density of woter! Finolly it cools like o dying fire to become o

2

blqck dworf.

Death of a big star

Small mass

-

a supernova

Large mass

a

@ I

yellow dwarf (like our Sun)

Lorger stors

A lorger stor burns hotter (o blue giont) ond runs out of fuel sooner. It exponds ond cools to o red supergiont. Loter it runs out of fuel, becomes unstoble, collopses, ond blows off its outer loyer in on enormous explosion colled o supernovo. During this explosion heovy elements (heovier thon iron, up to element92, uronium) ore produced ond thrown out ocross spoce, to become dust for moking new stors (see poge 152). Our plonet Eorth ond oll life on it ore mode from elements produced eorlier by stors. You ore stor moteriol! The core of o supernovo collopses to o very dense neutron stor (olso colled o pulsor). If this still hos o big moss it continues to collopse under its own grovity. The pull of grovity becomes so strong thot nothing con escope it, even light. It is o block hole.

blue giant

I

o

red giant red supergiant

+

o white dwarl

\-e\fi*fu

-$,:":'""''"-{€-

zrfl&'q a black dwarf

/\

neutron star

black hole

If you look corefully ot the night sky, you con see o foint bond of stors running ocross it. You ore looking edge-on ot our Goloxy.

It is o collection of obout 100 000 million stors Our Sun is just one of them, ploced somewhere neor the edge, in o spiral arm. !

The Goloxy is huge. It tokes 8 minutes for light to trovel from the Sun to Eorth; 4 yeors for light to trovel from the neorest stor; but 100 000 yeors for light to trovel ocross our Goloxy! 157

The Milky Woy is our goloxy, but it is not the only goloxy. Using telescopes ond rodio-telescopes, ostronomers hove found billions of other goloxies in the Universe! See olso poge 215. When we look of stors, we see them os they were, when the light left them. We ore looking into the post. Some stors ore 10 000 million light-yeors owoy, ond so we see them qs they were, 10 000 million yeors ogo ! So the Universe is even older thon this.

The 'whirlpool' galaxy, 37 million light-years away. lt is a 'spiral' galaxy, like ours, but others are different shapes.

The exponding Universe Astronomers con onolyse the spectrum of light from o stor (see poge 207). They hove found thot

the light from other goloxies always hos a red-shift This is like the pitch of o police-cor siren going lower os it roces post. It is colled o Doppler shift. This red-shift wos discovered by Edwin Hubble. He found thot the/arther awog o goloxy, the bigger the red-shift. This is colled Hubble's low. The simplest explonotion for this is: - oll the goloxies ore moving away from us, - the forthest goloxies ore moving owoy fostest. This meons thot the whole Universe is exponding, just like the dots on o bqlloon move forther oport os the bolloon exponds:

Imogine going bockwords in time, with the Universe (or the bolloon) shrinking. It meons thot ot some time in the post, ollthe moteriol in the Universe wos lumped together. This wos when it wos creqted (ond time begon) in the Big Bong. By meosuring the rote of exponsion of the Universe os it is now, ostronomers con colculqte thot the Big Bong wos olmost 14 000 million yeors ogo! The Universe hos been exponding ever since then, ogoinst the pull of grovity between the goloxies. If the totol omount of motter in the Universe wos big enough, the pull of grovity would stop the exponsion, ond the Universe would controct to o 'Big Crunch'. At present the evidence is not certoin, but it looks os though the Universe willkeep on exponding forever. Astronomers ore still seorching for better woys to meosure the totol moss of the Universe, including the 'dork motter' thot connot be seen. 158

Spectrum of light from our Sun. Light from a distant galaxy is red-shifted

-+

red shift

size ol universe

unt

time

Bang

Crunch

3 possib/e futures for the Universe

Are we olone, or is there other life in the Universe? Scientists ore looking for evidence in 3 woys:

. By analysing

meteorites thot foll to Eorth from spoce. They ore looking for fossils of bocterio, but no convincing evidence hos been found yet. By sending robots to explore our Solar System. A key condition for life (os we know it) is liquid woter. The most promising ploces for this ore: - Mors which moy hove hod woter in the post, - Europo, one of Jupiter's moons. It is covered in ice ond it moy hove liquid woter underneoth. The robots con look for life (eg. bocterio) or the chemicol chonges thot ore coused by life. By listening with radio telescopes (poge 215) for ony rodio signols from odvonced civilisotions like ours. This is the SETI project (Seqrch for Extro Terrestriol Intelligence). This seorch is mqde more difficult becouse there is o constont bockground 'noise' due to the microwove rodiotion thot wos left over from the Big Bong explosion.

.

.

Your ploce in the universe:

A Sojourner lander robot working on Mars

Radiolelescopes /islenrng for signals from space

(each drawing is 1000 times wider than the one belore it)

'I km=103m

1m

1OOO

km = 106 m

5

6

.

(__r-:-_/,r 7 / \ iols

m

the Solar System

'l012 m

lhe inner planets

1oe m

Summory Seismic woves tell us obout the Eorth's structure.

The movement of the Eorth ond Moon exploins: o doy, q yeor, the seosons, the Moon's phoses. The Solor System is the Sun ond 9 plonets. The Sun is o stor in our goloxy, the Milky Woy.

The Sun s energy comes from nucleor fusion. A stqr is formed from o nebulo, ond chonges os it gets older. The Universe is exponding, os shown by the red shift, which suggests the Big Bong theory.

159

Rockets astronauts and cargo

To escope the grovitotionol field of the Eorth, we use o rocket (see olso poge 85 ond poge 138) Use Newton's 3rd Low (poge 84) to exploin why

iuel (liquid hydrogen)

the bolloon moves:

-O neck tied

Balanced forces

+

liquid oxygen escaprng arr Resultant force

Which way does Prolessor Azlesser's machine-gun boat move?

expands at high speed

moves. Dlscuss

Explain how to

Is it sofe? A rocket is dongerous

- it is o controlled explosion

(i) faster (ii) tarther.

!

And interplonetory spoce is o hostile environment. It is extremely cold (tending to neor obsolute zero) ond it hos no otmosphere. It is full of rodiotion, including gommo-roys, X-roys ond cosmic roys (high-speed chorged porticles). Allthese con ionise ond domoge the humon body (see poge 214). When ostronouts ore in 'free foll' orbit round the Eorth (see poge 153), or for out in spoce, they feel weightless. Lock of grovity ollows the body to weoken during o long journey. Muscles get smoller, the heort becomes weoker, ond bones get thinner.

s An astronaut working in 'micro-gravily'

Artificiol grovity In o spoce stotion ostronouts con use exercise mochines for their muscles. A better solution might be to creote 'ortificiol grovity' with o rototing spoceship: In the rooms neor the rim, on ostronout will feel o force 'upwords' from the floor. This force is colled o centripetol force (see poge 70). It gives the impression of weight.

160

o 0

t o 0

Designing the spocecroft Discuss the design of o spoceship to toke 3 humons to Mors

shield m*4e er

VRAFT 2 sh

You will need to consider: . The length of time of the voyoge. . How to toke enough fuel, food ond woter.

,n

. How they willkeep worm. . The need to shield them from cosmic roys. . How to mointoin o stoble otmosphere. . The effect of low grovity on their heolth. . How they will lond on Mors. . How they will return to Eorth. e Sketch your design ideos ond odd notes.

piaQ

qt)

bbe's

vdear rcaclor ?)

Visit: www.nasa.gov or try

Google with 'spaceship lrrlars'

Living on Mors

Unmonned spocecroft (probes)

Look ot the doto on poge 151, ond reseorch obout Mors using the internet.

The only visitors on Mors so for hove been robots (see poge 159).

Discuss the design of o bose for living on Mors.

f g

h

odd notes.

Why is this? Whot ore the odvontoges?

Use your scientific knowledge to write obout o doy in the life of on ostronout living on Mors.

Robots con send bock doto to us by rodio. Discuss whether humon beings need to go, of enormous cost?

Sketch your ideos ond

761

) l.

7.

Questions Copy ond complete: q) The Eqrth hos . . loyers: b) Seismic P-woves ore . . . . woves ond con

gothrough....ond....

S-woves ore . . . . woves ond con go

2.

3.

through. . . . butnot. . . . Copy ond complete: o) A doy is the time for: b) A month is the time for: c) A yeor is the time for: d) The Eorth's oxis is tilted ot on ongle of . ond this couses our . . . . e) The nomes of the plonets, in ordeq ore: f) The shope of o plonet's orbit is on . . . .

.

Look ot poge 150 ond the toble on poge 151 Write two porogrophs obout eoch plonet to describe whot you would see, ond feel,lf you visited eoch one in o spoceship.

4.

5.

Copy ond complete: o) A sotellite in o higher orbit tokes . . . . time to complete one orbit. b) A sotellite or o plonet is held in . . . . by

f)

The Solor System wos formed by: Comets trovel in very . .. . orbits. The Sun is on ordinory . . . . , port of our . . . . colled the Milky Woy. Solor energy comes from nucleor . . . .

o.

. . . dwqrf stor. Loter it will chonge to o red . . . . , then o. . . . dworf, then o. . . . dworf. h) A lorger stor (o blue . . . .) will chonge to ored. . . . ond loterexplode in o. . . . leovingo. . . . stor, orq. . . . hole. i) The universe hos been . . . . since the

6. 162

is

orbit?

observotion sotellites ore ploced in different orbits. Exploin why.

:

9.

timeofthe...,

The light from distont goloxies shows o . . . . -shift due tothe . . . . effect. The forthest goloxies move . . . .

Exploin how the Solor System wos formed.

Describe whot will hoppen to our stor, the

Sun, in the fufure.

lO. Use the internet,

CD-ROMs or books to write

o porogroph obout eoch of the following: o) red gionts, b) white dworfs, c) supernovos, d) neutron stors (pulsors), e) block holes, f) quosors, g) cosmic microwoves, h) SETI.

ll.

Describe in your own words the evidence for the Big Bong theory for the origin of the Universe.

12.

Describe in your own words how scientists ore seorching for exho-terrestriol life.

13.

F-stimote the possibility of ET (extroterreshiol) life elsewhere in our goloxy, if there ore: - 100 000 000 000 stors in our goloxy, - 1 in 100 ore suitoble stqrs with plonets, - 1 in 10 plonets ore of o suitoble distonce from the stqr.

14.

Exploin why the journey for o humon to trovelto Mors willbe difficult.

o....force.

g) OurSun

j)

8. o) Whot is o geo-stotionory

b) Communicotion sotellites ond Eorth

On o sheet of groph poper, drow bor-chorts of o) the diqmeter of eoch plonet (see p.151), b) the density of eoch plonet. Whot pottern con you see? How does this fit with the nebulo theory of the origin of the Solor System?

c) d) e)

Look ot the toble on poge 151. o) Looking down column B ond column E, whot pottern do you notice? Why is this? b) Looking down columns B ond H, whot pottern do you see? Why is this? c) Looking down column D, whot do you notice? List the nomes of the plonets, divided into two cleor groups. d) Whot do you notice from column C? Why do ostronomers consider Pluto to be odd? e) From column G, how much would you weigh on i) Mercury ii) Jupiter? f) Whot is the connection between column D qnd columns C ond F? g) For on osteroid, estimote i) the distonce from the Sun, ii) the overoge temperofure, iii) the length of its 'yeor'.

15. Professor Messer

dreomed he wos living on on osteroid. Why wos his dreom o nightmore?

Further questions on poge 163.

Further questions on

3. 1.

Two types of shock woves ore P qnd S woves. Study the diogrom below. Cross Section of the

o)

I

Ecth ond beyond

2 hoursl 24 hours 1 week + weers16 months 1 year

Complete the following sentences by choosing o time from the obove list. i) The Moon moves oround (orbits) the t1l ii) The Eorth spins on its oxis once every t1l iii) The Eorth moves oround (orbits) the Sun once every .. . . t1] b) Use the doto on poge 151 to onswer the

Ecth

Eorthonceevery.....

.

following. i) Nome the plonet thqt tokes 44 times os long os Mors to orbit the Sun. tll ii) Nome the object whose diometer is qbout holf thot of Mercury. t1l iii) Whot is the distqnce in kilometres between Mors ond the Sun? t1]

x

shadow zone

zone

(wJEC)

4.

c

o) Nome the loyer morked X. ['l mark] b) Whqt is the exoct point of origin of on

eorthquoke colled? tll c) Whot ppe of wqve is on S wove? tll d) Exploin why the wove AB is curved. 12) e) No S woves trovelthrough the core. Whot does this tell us obout the structure of the Eorth's core? [1] (Nl)

2.

46-

I)O

ry/

earthquake

no transmission

X diagram Y diagram Z o) Describe ond exploin whot hoppens to the woves hovelling through the montle in diogrom X. t3l b) Describe ond exploin whot hoppens to the wove in diogrom Y Stote which ppe of wove is shown. t3l c) Exploin why the wove shown in diogrom Z does not hovel through the core. Stote which type of wove it is. l2l

F@\ o

@

@

@

o

@ wo

The smqll object close to Jupiter hod not been seen before. It wos loter nomed Io. o) Suggest q conclusion thqt Golileo could drow from his observotions. t1] b) Exploin how Golileo's observotions went ogoinst the belief thot oll heovenly bodies revolve oround the Eqrth. t3I c) Gqlileo published his findings in o book colled The Starry Mexenger. Why did Golileo publish his findings? [1] (Edex)

The diogroms show the poths of seismic woves through the Eorth's montle ond core.

diagram

In 1609, Golileo wos one of the first scientists to use o telescope. He used it to look of the plonet Jupiter. The 4 diogroms show whot he observed during one night:

5.

Describe how the orbit of q comet is different from the orbit of o [2] (OCR)

6.

This question is obout osteroids. o) How were osteroids tzl b) Whot is the evidence for post collisions of osteroids with the Eorth ond the Moon?[2]

plonet.

formed?

c) i) Exploin whot is meont by Neor Eorth Objects

(NEOs).

t2l

ii) How con they be detected? iii) Why moy they be o problem?

[4]

well,

163

Note: although a question may be marked (Edex) etc, it will often be relevant to other examinations as

l2l

Further questions on Eorth ond beyond

7.

9.

This question is obout sotellites ond spoce

explorotion. o) Communicqtion sotellites ore usuolly put into on orbit high obove the equotor colled o geostotionory orbit. i) Exploin whot is meont by o geostationary orbit. t1l ii) Whot is the odvontoge of such on orbit for communicoting from one port of the world to onother? tll iii) Whot is the time for o complete orbit of this sotellite? t1l iv) Why must it be ploced ot o porticulor height obove the Eorth's surfoce? t1l v) Why is the orbit obove the equotor? [1] b) Monitoring sotellites scon the whole Eorth eoch doy. By meons of o sketch ond o brief explonotion show how this is possible. t3l c) futronouts hove londed on the Moon ond it is hoped they will be oble to visit Mors in the future. Describe, briefly, two foctors which moke this difficult ond expensive. [4] (WJEC)

E.

o) Exploin the terms/requency and

red-shift.

b) Exploin how the omount of red-shift c)

d)

polar orbit

helps Rito to decide how for the X-roy source is from the Eorth. t1] When Rito mokes her observotions, she - uses o computer - tokes lots of reodings - repeots the meqsurements severol times. Exploin why she works this woy. t3l Suggest why it is likely thot Rito is right

theory?

A sotellite in stoble Eorth orbit moves ot o constont speed in o circulor orbit becouse there is o single force octing on it. o) Whot is the direction of this force? tll b) Whot is the couse of this force? t1l c) Whot is the effect of this force on the velocity of the sotellite? tll d) In which of the orbits shown obove would this force be bigger? Exploin the reoson for your onswer. 12) e) Exploin why the kinetic energy of the

0 g) 7&

constont. orbit. orbit.

sotellite remoins Suggest, with reosons, one use of o sotellite in o geostotionory Suggest, with reosons, one use of o sotellite in o polor [2]

12) 12]

(AOA)

t3l

thot the X-roys do not come from on intelligent source. l2l e) Ritq decides to publish her theory. Exploin how qnd why she would do this. l2l f) Rohul works in on observotory in Indio. He discovers onother X-roy source thqt floshes just like the one thot Rito found. Exploin why this neither proves nor disproves Rito's theory obout the new stor. l2l g) Whot else must hoppen before other scientists will be oble to occept Rito's

The diogrom below shows the orbits for two types of sotellite, o polor orbit ond o geostotionory orbit. geostationary orbit

Ritq works for o scientific institute colled 'Seqrch for Exkoterrestriol Intelligence' or SETI. Using o rodio telescope, she hos discovered o distont source of X-roys thot is floshing on qnd off. She meosures the frequency of the floshes ond the red-shift of the X-roys. Rito hos o theory thot this X-roy source is not proof of intelligent life in spoce, but o new kind of stor thot she describes with some new equotions.

12)

lO.

Stors begin their life in dense clouds of gos ond dust. Describe how they ore formed ond how they produce enerEg. [5] (OCR)

11.

Stors ore formed from mossive clouds of dust qnd goses in spoce. o) Whot force pulls the clouds of dust ond gos together to form stors? t1] b) Once formed, o stqr cqn hove o stoble life for billions of yeors. Describe the two moin forces of work in the stor during this period of stobility. l2l c) Whot hoppens to this stor once this stoble period is over? t4l

d)

Suggest whot might then hoppen to o plonet close to this stor. [1] (AoA)

Further questions on Eorth ond beyond

12. The toble gives informotion obout Satellite name Purpose

orbit height

3 sotellites.

16.

Approximate

above surface orbit time

Hubble space telescope

to observe stars

612km

1.5 h

NOAA-17

to monitor the Earth and oceans

833 km

1.7 h

lntelsat906

communications

36 000 km

24h

o) How does the orbit time for

period.

q sotellite

chonge with distonce from the Eorth? [1]

b) i) How mony times does Intelsqt-906 orbit the Eorth in one doy? tll ii) Why is this importont for o geostotionory sotellite? t1l iii) Stote two uses for o communicotions sotellite like Intelsot-906. l2l c) NOAA-17

tokes 100 minutes to orbit. How mony orbits does it moke in one doy? tl l ii) Exploin how its polor orbit qllows it to monitor oll the Eorth's oceqns. l2l d) The Hubble telescope wos used to study the foint light from distont stors. Suggest why it wos ploced in orbit just obove the Eorth's otmosphere. [2] (Edex)

i)

13. Telescopes

con be used on Eorth or in spoce (see poge 215). o) Whot ore the odvontoges ond disodvontoges of using telescopes in spoce? t4] b) How do different telescopes help us to moke deductions obout the Universe? lal

14. This is o question obout spqce trovel. o) Discuss ond exploin the problems involved in putting on ostronout on Mors. l4l b) How ore scientists devising woys to

problems? 15. o) Whot is meont by red-shift? b) Whot is o light yeor? overcome these

Stors do not stoy the some forever. o) Over billions of yeors the omount of hydrogen in o stor decreoses. Why? tll b) Describe how o mossive stor (qt leost five times bigger thon the Sun) will chonge ot the end of the moin stoble t4l To gain t'ull marks you should write your ideas in good English. Put them in a sensible order and use the correct scientit'ic words. c) The inner plonets of the Solor System contoin otoms of the heoviest elements. i) Where did these otoms come from? t1l ii) Whot does this tell us obout the oge of the Solor System compored with mony of the stors in the Universe? [1]

t4]

tll tll

c) A goloxy is 1200 million light yeors owoy from us. It is moving owoy from us ot 30 000 km/s, which is one-tenth (0.1) of the speed of light. Assuming thqt its speed hos been constqnt ond thqt oll the motter in the Universe wos originolly in one ploce, how long hos it token for the goloxy ond us to be this for oport? 12) d) Whot is the significonce of this volue? [1] (ocR)

(AOA)

17. o) Whot

is meont by the term

used in

ostronomy?

b) Exploin how the red-shift

red-shift,

os 121

helps us to find

out more obout

i) ii)

goloxies the universe.

l2l

[2]

(rOex)

18. o) How b)

does the 'big bong' theory of the Universe occount for its creotion? l2l Outline the evidence which supports the

'big bong'

19.

theory.

[3]

(AOA)

to hove formed from clouds of dust ond gos. t3l b) During the stoble period of o stor, high temperofures creote forces which oct outwords. These forces ore bolonced by its inword grovitotionol forces. Briefly describe the circumstonces whereby eoch of the following will occur. i) Our Sun becomes o red giont. L2) ii) A red giont becomes o white o) Exploin how stors ore thought

dworf. fusion.

c) Briefly describe the process

12)

of nucleor 121

d) Whot evidence is there to suggest thot

the Eorth ond the other plonets in the Solor System were formed from moteriql produced when eorlier stors

exploded? is exponding?

t3l

e) Whot evidence is there thot the Universe

[4]

(Edex)

165

- Waueq

chapter 27

{es !{f, vF6 !

Energy is often moved from one ploce to onother by woves. You con see this if you throw q stone into o pond - woves spreod out from the splosh ond soon the woter of the edge of the pond moves os it gets some of the energy of the splosh. There ore two kinds of woves:

D Trqnsverse wqves fueriment 27.7 To see o fuonwerse wqve, sfuetch o long 'slinky'

---->

spring olong o smooth bench or floor ond then give one end o quick wiggle at fight ongles to the spring. Whot hoppens? You con see thqt energy is moving down the spring olthough eoch bit of the spring is only vibroting. It is vibroting ot right ongles (transoersely) to the spring.

In this chopter we will be investigoting tronsverse woter woves. Light woves ore qlso konsverse woves.

F

Longitudinol wqves

Experiment2T.2 Sbetch the some 'slinkgr' spring olong the floor ond this time give it o quick wiggle to ond fro along thelength of the spring. Whot hoppens?

If you look closely you con see thot ogoin, enerEg is possing down the spring olthough eoch bit of the spring is only vibroting (or bscilloting'). It is vibroting lengthwoys or longitudinally. We will investigote longitudinolwoves in chopter 29 (po3e 224). Sound woves ore longitudinol woves.

t6

move bar l up and down

I

We con study tronsverse woves by using woter woves in o ripple tonk.

WAVES

Experiment 21.3 Put some smoll pieces of cork on the woter in o ripple tonk (or your both) ond move the wooden bor up ond down to moke stroight woves. As the woves move down the ripple tonk, which woy do the pieces of cork move? Are these tronsverse or longitudinol woves? wavelength

Here is o side view of o tronsverse woter wove:

The high ports ore colled crests ond the low ports ore colled troughs. The distonce morked o is colled the omplitude. It is the height of o crest obove the overoge woter level (or the depth of o trough below it).

a

o wavelength

The distonce between two successive crests (or troughs) is colled the wovelength. If the wooden bor (or o piece of cork) in the experiment moves up ond down through tr,r,,o complete vibrotions in eoch second, then the frequency of the vibrotions is two cycles per second. This is olso written os 2 hertz or 2Hz.

The time period of o wove, T, is the time token to produce one complete wove. It is connected to the frequency by:

Time period,

A wove which vibrotes only up ond down like this is soid to be polarised verticolly. Experiment 21.4 Move the wooden bor up ond down of o low frequency first ond then ot o higher frequency. Whot do you notice obout the speed of the woves ond their wovelength? You should be oble to see thot: o) the speed stoys the some ond b) when the frequency increoses, the

wovelength decreoses: long wavelength

low frequency

T:

1

frequencVJ

Example A wove hos o frequency of 2 second) ond o wovelength of Whot is the speed of the inI

In eoch second, two woves ore of length 10 cm. Therefore the wove hovels forword second. Thot is, the speed of the In foct, for ony wove

short wavelength

high Irequency

wove speed

(m/s)

: frequency x (Hz)

wovelength (m)

L67

dipper straight mirror

Experiment

21.5

Reflection at a straight mirror

Ploce o stoight metol or plostic borrier in the tonk so thot it reflects the woves (like o mirror). Send single woves ocross the ripple tonk qnd look corefully for the woves reflected off the 'mirror'.

Whot do you notice obout the ongle ot which the wovefront hits the mirror ond the ongle ot which the reflected wqve leoves the mirror? Whot do you know obout the ongles when light is reflected from o mirror? (See poge 177.)

wavefronts moving forward

Experiment 27.6 Reflection at a curued mirror Ploce o curved borrier in the ripple tonk. Agoin send single woves ocross the tonk ond look corefully for the reflected woves. Whot do you see? The woves ore conerged to o point colled the Jocus (See olso poge 181).

fo

Experiment 27.7 Repeot experiment 21.6 with the curved mirror turned round the other woy. Con you see thot the woves ore spreod out or diuerged by the mirror? (See olso poge 181.)

F.

Experiment 27.8 Refraction Ploce o thick sheet of gloss in the tonk ond odjust the depth so thot the woter is very shollow over the gloss sheet. Agoin send woves ocross the tonk ond look of the woves when they bqvelthrough the shollow woter.

Whot do you see? Whot hoppens to: o) the speed ond b) the wovelength of the woves when they move into the shollow woter? Experiment 21.9 Refraction Now ploce the sheet of gloss of on ongle ond repeot the experiment.

Whot hoppens to the direction of the woves os they slow down in the shollow woter? When woves chonge their speed ond direction like this, we soy they hove been refracted. Sound ond light woves con olso be refrocted (see poge 184). 168

ll

F' Diffroction Do woves chonge shope os they go through o gop in q woll? Experim Use o woter to o

Wide gap

send stroight wqves ocross gop between two borriers.

Look of the woves ofter they hove possed through the gop. Are they still perfectly stroight? Experiment 21.1.1. Narrow gap Now send stoight wovss through o norrow gop.

Whot hoppens to the shope of the woves? In the first diogrom, with o wide gop, the woves trovel olmost stroight on ('rectilineor propogotion'). There is o cleor shodow behind eoch borrier. In the second diogrom, the woves spreod out ofter possing through the gop. This is colled dffiaction. Diffroction is most obvious when the width of the gop is obout the some size os lhe waoelength of the woves. The photogroph shows some seo woves possing through into o horbour. If you look corefully you con see the woves spreoding out in the horbour:

Sound woves ore diffrocted through doorwoys,

becouse the wovelengths of sounds ore qbout the some size os doorwoys. This is one reoson why you con heor people without seeing them.

is seen to be diffrocted only if it posses through o very norrow slit. This shows us thot lighf is a waue motion, with o very smoll wovelength.

Light

Rodio ond TV woves con olso be diffrocted. (Your teocher moy be oble to show you this with 3 cm rodor woves.)

This offects the lives of people living neor hills: Rodio woves (longer wovelength) con diffroct round the hillto be received by the house, but TV woves (short wovelength) do not bend qs much. The house will hove poor TV reception.

r l! TV and

radio transmitter

lll l

l

ll Il

|

,,

*uuu,

radio waves

169

Summory Woves move energy from one ploce to qnother (olthough eoch port of the wove only vibrates to qnd fro). In transuerse woves (like woter woves ond light woves) this vibrotion is ot right ongles to the direction of the wove.

Wqves cqn be refTected ond refrocted.

ln longitudinol woves (like sound woves) the vibrotion is olong the direction of the wqves.

Woves cqn be diffrocted through o norrow opening.

tr 1.

Questions Copy ond complete: o) If the vibrotions of o wove ore ot right ongles to the direction of the wove, it is colled o . . . . wove. If the vibrotions of o wove ore olong the direction of the wove, it is colled o. . . . wove. A woter wove is o. . . . wove ond osoundwove is o. . . wove. b) The wovelength of o wove is the . . . . between two successive . . . . (or. . . .). The frequency of o vibrqtion is meosured

in....or.....

2. a) The speed of o wove (meosured in . . . .) is

The shorter the wovelength, the higher the frequency. For oll woves: speed

:

frequency

x

wovelength

6. 'l0cm-

The diogrom shows some woves in o ripple tonk trovelling ot o speed of 6 cm/s. Whot is o) the wovelength b) the frequency? 7

.

The diogrom shows stroight woves opprooching o stroight borrier ot on ongle of 45". Drow o diogrom to show how the woves ore reflected from the 'mirror'.

equolto the frequency (meosured in . . . .) multiplied by the . . . . (meosured in ). b) When woves trovelthrough o norrow gop in o woll, they ore This effect is most obvious when the . . . . of the gop is similor to the

3. 4.

5.

A certoin sound wove hos o frequency of 170 hertz (cycles per second) ond o wovelength of 2 metres. Whot is the speed of sound?

8. A vibroting source S produces circulor st2g45

I

A rodio stotion produces rodio woves of frequency 200 000 hertz (cycles per second) ond o wovelength of 1500 mehes. o) Whot is the speed of rodio woves? b) Another stotion produces woves ot 500 kHz. Whot is their wovelength? The time period of o wove, ?, is the time token to produce one complete wqve. If the frequency of o wove is2Hz, whot is its time period?

I

45 mm

o) Copy ond

complete the diogrom to show how crests 4 ond 5 ore reflected. b) Whot is the wovelength of these woves? c) If their speed is 60 mm/s, whot is the frequency?

Further questions on poge 236. 170

woter

woves neor o stroight reflector:

chapter 22

I

l[d I Visible light is o form of energy thot we cqn detect

with our eyes. Objects thot produce their own visible light ore colled luminous sources (for exomple, the Sun, other stors, lomps, televisions, glow-worms).

Other objects ore illuminatedby this light ond reflect it into our eyes (for exomple, this poge ond most objects in this room, ond the Moon).

Light woves

*

rr*t

rr'r'r --------+

Light is o wove motion, rother like the woter woves thot you con see on o pond or in o ripple tonk: The light thot we con see hos o urouelength of only of o millimetre! obout

^lo The speed of light Light trovels ot o very high speed - obout o million times foster thon the speed of sound. Light tokes only 8 minutes to trovel from the Sun, - o fost cor would toke obout 150 yeqrs! The speed of light in oir is 300 million metres per second. Alight-year is o distonce - how for is it?

Roys of light For experiments with visible light, we often use o very norrow beom of light colled oray. electric lamp

Experiment 22.7 Use o ragbox to shine o norrow beom of light ocross a piece of poper.

Whot do you notice obout the edges of the beom? Why con't you see round corners?

Light trovels in stroight lines. It is 'rectilineor'. 777

F

Shodows

Shodows ore formed when some roys of light continue to trovel in stroight lines, while other roys ore stopped by on object. The kind of shodow thot is formed depends on the size of the source of light:

I

Shodow due to o smqll source of light Experiment 22.2 Hold o toble-tennis boll between o smoll electric lomp ond o white scaeen.

screen

dark shadow (umbra)

ball small lamp

Con you see o dork shodow with shorp edges? This shodow is colled the umbra.

2 Shodow due to o lorge source of light Experiment 22.3 Chonge the smoll lomp for o lorge peorl lomp.

large lamp

screen

ball

umbra penumbra

Does this shodow still hove shorp edges? ollthe shodow equolly dork?

Is

The dqrk centre of the shodow is still colled the umbro. No light from the lomp reoches the umbro. It is surrounded by o fuzzy portiol shodow or penumbra. The penumbro vories in brightness - from very dork neor the umbrq (where not much light con reoch it) to very bright ot the outer port where more light con reoch Which kind of shodow is formed on o cleor sunny doy? Which kind of shodow is formed on o cloudy doy? t72

How many things can you deduce from this photo?

Eclipses occur becouse the Moon ond the Eorth cost lorge shodows

1 Eclipse of the Sun (solor eclipse) This hoppens when the Moon comes between the Sun ond the Eorth, so thot the Eorth (in the shodow) dorkens during the doy. At the time of totol eclipse, only the flomes of the outer edge of the Sun con be seen.

Earth

Moon SUN (not to scale)

Experiment 22.4 Set up o model of on eclipse of the Sun (in o dorkened room) using o lorge peorl lomp for the Sun, o toble-tennis boll for the Moon ond o white footbollfor the Eorth. Totol eclipses of the Sun do not occur often. After the eclipse in August \999, the next totol eclipse in Britoin is on 7 October 2735.

2 Eclipse of the Moon (lunor eclipse) This hoppens when the Moon moves into the Eorth's shodow. When the Moon is in the Eorth's umbro, no sunlight con reoch the Moon ond it is seen to go dork. /

Earth

SUN

(not to scale)

Experiment 22.5 Use the some opporofus os in experimenlZ2.4 to show on eclipse of the Moon. Eclipses of the Moon con be seen much more often thon eclipses of

the Sun. Tiy to find out the dote of the next eclipse of the Moon to be seen from where you live. How long does it toke for the Moon to move once round the Eorth?

partral

773

closed cardboard box

A simple pinhole comero consists of o closed box with o screen of one end qnd o smoll pinhole ot the other end. It uses the foct thot the light going in through the pinhole trovels in stroight lines to the screen. The diogrom shows one woy to moke o pinhole cqmero.

hole cut out, then covered with kitchen foil

hole made with a pin

Experiment 22.6 Use o pinhole c-->=,>-=>-/

lomp. A

constont current of 3 A posses round the circuit.

o) b)

Exploin whot hoppens to the energy of electrons os they flow in the lomp wire. [4] How much energy is tronsferred by the

lomp in 20

seconds?

l3l

o) Copy it ond show the positions ond signs of the induced chorges on the oircroft. [2]

b) Exploin, in terms of the movement of

(AOA)

electrons, the dishibution of the chorges you hove shown. t3l c) Whot would hoppen to the induced chorges when the oircroft flies owoy from the cloud? [1] (Edex)

20" A lomp is morked

'3.0 V 0.2 N. Writing down the formulo thot you use in eoch port ond, showing your working, colculote

o)

its resistonce when it is working of its

normolbrightness,

i3]

b) its power, [3] c) the chorge possing through it in 600 s, [3] d) the energy it tronsfers in 10 minutes. i3l

?4".

o) woollen cloth

charged balloon

(wJEC)

21"

Shoun is cutting o lorge oreo of gross with on electric mower. He is worried thot he might cut through the moins coble of the mower. He decides to use o residuol current circuit breqker (RCCB) os well os o fuse. o) Describe how the fuse gives protection. [2] b) Exploin how on RCCB provides

odditionolprotection.

c) The toble shows how different currents

A bolloon, ofter rubbing with o woollen cloth, is found to be negotively chorged ond is othocted to the cloth. Exploin: i) why the bolloon becomes negotively chorged when it is rubbed, 121 ii) why the bolloon is othocted to the

cloth.

tzl

b) Nome one procticol

electricity. c) i) Stote one situotion where stotic electricity is dongerous.

ore likely to offect people when they receive on electric shock. Effect on people Current in mA '1

5 '10

100

ii)

none

tingling etfect could become harmful probably {atal

Shoun's RCCB switches off in just 0.02 s when it detects residuol current of 30 mA. Use the doto in both columns of the toble to suggest how effectively this RCCB would protect Shoun. [2] (Edex)

tll

use of stotic

Zli.

t1l

tll

Exploin whot precoutions con be token to reduce the donger. tll (wJEC)

Electrostotic point sproying is the most efficient woy of sproying cqr bodies. o) Exploin why the point drops spreod out

ofterbecomingnegotively-chorged.

11]

b) Exploin why the cor body ponel is given on opposite (positive)

c)

chorge.

t1l

Suggest two reosons why this method of

sproying is very

efficient.

l2l 279

}--.

Whqt is the quickest woy of picking up some pins thot hove been spilt on the floor? If o mognet is used, pins stick to the ends or poles of the mognet. Con you think of ony other uses for mognets? Experiment 34.7 Ploce o bor mognet in o poper stirrup os shown ond hong it from o wooden toble or o wooden retort stond.

cotton

Leove it honging until it stops moving. From o mop or from the position of the Sun, find out which direction is North. Which direction does the mognet point to? You hove mode o composs . A freely moving mognet comes to

rest pointing roughly North-South. The end pointing North is colled the Norfh-seeking pole or Norfh pole (N-pole) of the mognet. The other end is the Soufh-seeking pole (S-pole).

paper stirrup

Experiment 34.2 Mork the N-pole of the mognet in experiment 34.1. Then toke thqt mognet well owoy ond repeot the experiment with onother mognet.

Now bring the N-pole of the first mognet close to the N-pole of the honging mognet. Whot hoppens? Whot hoppens if you bring the S-poles close together? Whot hoppens if you bring o N-pole neor o S-pole? We find

thot: Like poles repel eoch other. Unlike poles ottroct eqch other.

A// physics students shou/d learn well

Ihis scientific fact. Like magnetic poles repel And unlike poles altract.

\

) Moking o mognet

A.

Experiment 34.3 Mork one end of o piece of unmognetised steel. Check thot it is not mognetised by dipping it into o dish of smoll iron noils. Stroke the piece of steel with the N-pole of o mognet. Stort ot the morked end of the steel ond lift the mognet cleor ot the end of eoch stroke. Do this obout L0 times in the some direction.

., --1--0.4 0.3 0.4 0.25 0.3

I I o.2 0.25 a