Unit 5Work, Energy and Power

l. A 64.8 kg hiker starts at an elevation of 1 500 mand climbs to the top of a 2 500 m peak. Whatis the minimum power to be applied by the hikerif he takes 6 hours to complete this climb? (Take

acceleration due to gravity as L0 m s-2)

A 30W B 180Wc 1080w D 6480w ( )

2. An object of mass 20 kg is pulled up a slope of15 m long.The frictional force between the object

and the slope is 30 N. The minimum work done

by the pulling force is

6 (o

5m

5. A force of 30 N acting on an object causes it tomove along a horizottal surface at a constantvelocity of 3 m s 1. The power developed is

30N

A 90Wc 27W

6. A ball is released from rest from one side of a

smooth curved rail as shown in the diagram below.Neglecting air resistance and friction, which ofthe following statements about the motion of theball is/are correct?

Ball

A C

I The speed of the ball at C is the same as thatat point A.

II The ball has a maximum potential energy at

point B.III The ball would not rise to a level higher than

point D.

A III only B I and IIIC IIandIII D I,IIandIII

()

7 . At electric heater has a power output of 300 W.What is the quantity of heat generated in 30

minutes?A 1oJ B lsoJc e oooJ D s4o oooJ ( )

B 33WD 10W

3. A 0.2 kg mass is thrown vertically upwards so

that it reaches a height of3 m above the point ofprojection. What is its kinetic etergy when it is

2 m above the point ofprojection?

OI

A 450Jc 1450J

B 1000JD 3000J

1m

point of projection

()

()

B

3m

A 1oJc4I

BD

6J2l

4. What are the energy changes in hydroelectricpower production?A electrical + potential - kineticB electrical - kinetic + potentialC potential - kinetic - electricalD kinetic + potential - electrical ( )

()

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8. The diagram below shows two blocks P and Q,initially stationary and connected to each otherby a taut string passing over a smooth pulley. The

system is released from rest and block Qmovesvertically downwards from position A to B.

Which of the following is correct?

pulley

A

1i. A container is lifted vertically with a force of10 000 N from the wharf onto the deck of a ship,a height of 5 m.

crane

t container

m

wharf wharf

What is the work done?

A sooooN B 50000Jc looooJ D 2500J

72. A man of mass 60 kg takes l minute to run uP

stairs from X to Y as shown in the diagram below.

The man has a working power of

m

8m

T

A

B

A Potential energy of the block Q:at A = Kineticenergy of block Qat B.

B Loss in potential energy of the whole system

= Gain in kinetic energy of the whole system

+ work done against friction due to block P

moving against a rough surface.

C Loss in potential energy of the whole system

= Gain in kinetic energy of the system.

D Loss of potential energy of block Q= Gainin potential energy ofblock P ( )

9. A block slides down from rest at point A.Assuming the frictional force is negligible, whatis the speed of the block when it reaches B if ittravels 4.0 m along the slope?

3.0 m

slope

B

A 7.7 ms'1B 8.5 m s-1

C 9.0ms1D cannot be determined

10. A girl weighing 400 N runs uP a flight of stairs

of vertical height 5 m in 4 s. What is her gain inpotential energy and the power developed?

Gain in potential Powerenergy developed

A 1600J 40owB 2000J 50owc 16 000J 4000wD 20 oo0J sooow ( )

13. A weight is to be lifted from a level X to a higherlevel Y along different paths labelled L, M and

N.

A 140Wc 80w

B lOOWD 60W

L

Which path requires the least amount of work?

A Path LB Path MC Path ND An equal amount of work is done along the

three paths. ( )

NM

X

()

()

()

ship

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14. The diagrams below show a man (i) about toclimb a ladder, (ii) climbing up a ladder and (iii)halfiaray down after falling offthe ladder.Which of the following is correct?

(i) (ii) (iii)

17. At object falls freely from rest (point X) to theground (point Z). What is the ratio of the kineticenergy of the object at Y to its kinetic energy atZ?

X

Z

o

o

o

a

a

Between (i)and (ii)

A Gain of potentialenerg"y

B Loss of potentialenergy

C Gain of potentialenerg'y

D Loss of kineticenergy

Between (ii)and (iii)

Gain of kineticenergy

Gain of kineticenergy

Loss of kineticenerg'y

Gain of kineticenergy

AC

1

41

1.

B l:2D 2:-J.

.41

15. A wooden block of mass of 30 kg is pulled up a

rough inclined plane at a constant speed by a forceof 70 N parallel to the plane. When the distancemoved along the plane is 12 m, the increase inheight is 1.0 m (see diagram below).

1'o

30

What is the work done against friction?A 300J B 5401c 360J D 840J

16. A toy boat was propelled steadily from rest toreach a constant speed of2.0 m s 1in ten seconds.During this time, there is an average waterresistance of 3.0 N acting between the base ofthe boat and the water. Calculate the rate ofworkdone against water resistance.A 3.00 w B 6.00 wc 30.0w D 60.0w ( )

18. A dry battery can deliver 3000 J of energy to a

small 2 W electric motor before the battery is

exhausted. For how many minutes does the motorrun?A 1500 B 100cso D2s ( )

19. A girl weighing 500 N runs up a fight of steps

6 m in height in 3 s. What average power is thegirl developing?A 250W B 1000Wc 1500w D 2000w ( )

20. An object is dropped from a tower 50 m high.When it is 10 m from the ground what is theratio of the body's potential energy to its kineticenergy? (Neglect air resistance)A 1:5 B 1:4C1.:2 D4 1 ( )

21,. From which source would the release of energy beaffected by a change in the Earth's gravitationalfield?A Chemical B GeothermalC Hydroelectric D Nuclear ( )

22. A car of mass mhas an engine which can deliverpower P. What is the minimum time in whichthe car can be accelerated from rest to a speed z?

^# B *^ mt} 2Pc 2P D# ( )

1.0 m\z ((\

()

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23. the diagram shows a set of five steps. A student 28of mass 50 kg, runs up the steps. What is thestudent's gain in gravitational potential energy?

k=10Nkg')

40cm'

25 cm

A 5"(25+40)x50J5,(25+40)x50

Walking up to the top of a steep hill usually seems

to be easier if a zigzag path is used instead ofclimbing by the steepest route. Why is this so?

A Less energyis needed.

B Less friction has to be overcome.C Less power is needed.

D Less time is needed. ( )

29. A force of 20 N pushes a block of wood of mass

1 kg, causing it to move at a constant speed of4 m s-l.

+4mls

What is the power required to push the wood?A 5W B 7.5Wc40w D80w ( )

30. A car cruises along a frictionless track of a rollercoaster.

At which point A, B, C, or D is the car cruisingat its maximum speed? ( )

31. A stone of weight 100 N is thrown verticallyupwards from the ground with an initial kineticenergy of160J. Ignoring air resistance, the kineticenerg'y, the gain in potential energy and the workdone against gravity when the stone is at 1.00 mabove the ground will be

Kinetic Gainin Workenergy potentialenergy done

A 60J 60J 60JB 60J 100J 60Jc 60J 100J 100JD 100J 160J 100J

()

32. When a car accelerates up a mountain, itA gains both kinetic energy and potential energy.

B gains kinetic energy but loses potential energy.

C gains potential energy but loses kinetic energy.

D loses both kinetic and potential energy.()

B J1005"25*50x10

C100

5"25"50D100 I

A 0.79 m s-1

C 0.62 m s-1

I)

24. A 60-watt light bulb turns half the electricalenergy it uses into thermal energy. How muchthermal energy does it give out in l-0 seconds?

A 60I B 6ooJc30oJ DoJ ( )

25. Which ofthe following expressions defines power?A force , distance moved in the direction of the

forceB force , accelerationC force * perpendicular distance from the pivotD force x distance moved in the direction of the

force per unit time ( )

26. Water is pumped out a tank by an electric motorat the rate of 150 kg s-1. The speed of the water as

it leaves the tank is 20 m s-1. What is the power ofthis electric motor?A 18kW B 27kWc 30kw D 45kw ( )

27. On braking, 500 kJ of thermal energ'y wereproduced when a vehicle of total mass 1600 kgwas brought to rest on a level road. The speed ofthe vehicle just before the brakes were appliedwas

A

B

D

B 25ms1D 62.5 m s1 ()

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33. On braking, 250 kJ of heat was produced whena vehicle of total mass 1000 kg was brought torest on a level road. The speed of the vehicle justbefore the brakes were applied was

36. A fat man of mass 90 kg and a thin man of mass

40 kg take 5 seconds each to run up a flight ofsteps measuring 10 m in height. Calculate thepower generated by each man. (Take g = 10 m s-')

Fat man Thin manA 900w 400wB 1800W 800Wc 9000w 4000wD 180W 80W ( )

37. An object is lifted at constant speed through a

distance ofl metres above the ground in / seconds.The potential energy gained by the object is equalto

-.

A work done on the objectB work done on the object minus kinetic

energ'y of the objectC force acting on the objectD work done x time ( )

38. A pendulum bob of mass, m, attached to a lightstring is released from rest at position P. It makes

one complete oscillation form position P to S andback to P again. The speed of the bob at its lowestpoint Qis ?rl m s 1.

o R

What will be the kinetic energy of the bob atpoint R?

zefo

mzu2

A 4ms-1C 250 m s-1

B 22.4ms1D 500ms-1 ( )

34. A crate is pushed 10 m along a horizontal surface

by a force of 80 N. The frictional force opposingthe motion is 60 N.How much of the work done is converted intokinetic energy and internal energ'y ofthe crate?

Internal erergy / J Kinetic energy / IA 200 600B 200 800c 600 200D 600 800

()

35. Tom, Dick, Ali and Raju stand on concrete steps

as shown. Each person carries an equally sizedsphere of different mass. They drop the spheres

onto a smooth level sand pit.

paiycarfles100 g

Aticarries150 g

Dickcarries200 g

Tomcarries300 g

Whose sphere would sink the deepest in thesand?

A Tom's B Dick'sC AIit D Raju's ( )

SP

rnlD21

z

AB

C

D 1 mzu2 ( )more than zero but less than ,Danya

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39. A ball is allowed to ro11 along an undulating plane 43

as shown. It is at rest at A.

AE

D

Neglecting air resistance and assuming the planeis smooth, which of the following statements is

true?A Kinetic energy at B is less than kinetic energy

at E.B Potential energy at A is greater than kinetic

energy at D.C Kinetic etergy at B equals kinetic energy at C.D Potential energy at B is greater than potential

energy at C. ( )

40. A motor rated 50 W accelerates an object alonga horizontal surface with a constant speed of2 m s-1 against a frictional force of magnitude of19 N. What is the efficiency of the motor?A 38 o/o B 50 o/o

C 760/o D 100o/o ( )

41. An object of mass m passes at point X with a

velocity v and slides up a frictionless incline planeto stop at Y which is at a height D above X. Asecond object of the same mass passes X with a

-lvelociry of ,r,.To what height will it rise?

A meteorite of mass 10 kg enters the Earth'satmosphere and decelerates from 99 m s 1 to1.0 m s-1 due to air friction. What is theapproximate work done against air friction?A e80J B ee00Jc s7 600J D 4e 000J ( )

44. Which of the following about energy is false?

A Energy has to be supplied when a man bendsan iron bar

B Energy has to be supplied when a chairsupports a man sitting on it.

C Energy has to be supplied when a helicopterlifts a passenger offthe ground.

D Energy has to be supplied when a boy swingshis arms during exercise. ( )

45. Which of the following is equivalent to the unitof power?

A Jr' B NmC kgmz5-: D kgm2s-3 ( )

46. A toy car K moving with a speed of 30 m s-1 has

kinetic energy of 900 J. Another identical toy carL moves with a speed of 15 m s-1. What is thekinetic energy of the toy car L?A 22sI B 4s0Jc e00J D 18ooJ ( )

47. 'Ihevariation ofthe power of a machine with timeis as given below.

rAlV

'10 12 time/s

What is the work supplied by the machine?A 0J B 240Jc 360J D 480J ( )

h

4X

Blt,*u

ol,.i,

42. An electric motor runs with a steady input of250 V ard 4 A while raising a load of 1000 N.Assuming that the motor and transmission is

100 0/o efficient, how much time is taken to lift theload vertically through a distance of 10 m?A 1s B 1.5sC 4s D 10s ( )

)

0

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48. A ball bearing is placed at point X on a smoothtrack as shown in the diagram below. The ballmoves from rest down the track and passes a

point Y which is 10 m below point X. Take

8= 10 ms-2'

X

10m

IFind the speed of the ball at point Y.A 10.0ms1 B 14.1 ms-1

C 100ms1 D 141ms'1

49. Kinetic energyis converted to electrical energy inA an electric motor.B a electromagnetic relay.

C a galvanometer.D adynamo. ( )

50. An object of mass 0.3 kg moving at a constantspeed has a 21.6 J of kinetic energy. At whatspeed is it moving?A 144ms-1 B 72ms-1C 12ms-1 D 6.5ms-1 ( )

51. A ball is released from a height h above a table.Assuming that air resistance is negligible and50o/o of its kinetic energy is lost at each bounce,what will be the height reached after the second

bounce?

52. Ar object has kinetic energy of 10 J at a certaininstant. If an opposing force of 5 N acts on it,what is the distance it will travel before comingto rest?

A 2m B 5mC10m D50m ( )

53. The diagram below shows a box pulled by a

5 N force. A frictional force of 3 N acts in theopposite direction. What is the work done by thebox against friction if it moves 5 m?

forcefrictionalforce of 3 N

5

hBZD+

h:^lzb4

A

C

B 1oJD 2oJ

0J1sJ

AC

()

()

()

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