Unit 3Forces, Work, Energy, Powerand Pressure

Multiple-choice Qrestions

l. The graph below shows how the velocity varieswith time for a given body.

velocity

Which of the following statement is/are true?

I The resultant force acting on the body is

rfever zeto.II The forces acting on the body is never

constant for any period of time.III The object is never at rest.

A I and II B I, II and IIIC IIandIII D IIIonly ( )

2. A body is moving in a straight line under theaction of a constant force. What change willoccur to the body?A Change in acceleration

B Change in inertiaC Change in kinetic energyD Change in mass ( )

3. A constant horizontal force is applied to a bodyinitially at rest on a smooth horizontal table.Which of the following quantities will not change

during the application of the force?A The displacement of the bodyB The velocity of the bodyC The acceleration ofthe bodyD The kinetic energy ofthe body ( )

4. Two forces act on an object. The forces are Xand Y. The object is moving in the horizontaldirection. Which values of A and B would cause

the object to move with constant speed?

X

0

xA 2ONB IOONC 80ND IOON

Yr00 N20N80N80N ()

5. An object is accelerating under the influence of aforce F, on a frictionless surface. A while later, an

opposing force { of the same magnitude acts onit. What will happen to the object?

A The object will slow down.B The object will move at constant velocity.C The object will come to rest immediately

after the opposing force acts on it.D The object will move in the opposite direction.

()

6. The velocity-time graph of a particle moving in astraight line is shown below.

velocity/m s-1

time/s

What may be deduced about the resultant force

acting on the particle during the first 2 s and after2s?

F.1

20

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7. A box of mass 2 kg moves with a constant speed

of 0. I m s-r when a force of 6 N is applied.

0.1 m s-1+

6N

What is the frictional force acting on the box?

A ON B 2Nc 3N D 6N ( )

8. An astronaut's boots weigh 100 N on Earth where

the acceleration of free fall is l0 m s-2. How much

will they weigh on Mars where the acceleration

of free fall is 4 m s'?A 25N B 4ONc r00N D 400N ( )

9. When a block of mass 3 kg was pushed along a

surface with a force of 3 N, it moved with constantspeed.

When the same block was pushed with a force of9 N,what happened to the block?

A It moved with a constant speed of 2 m s-r.

B It moved with a constant speed of 3 m s-r.

C It moved with a constant acceleration of 2 m s-2.

D It moved with a constant acceleration of 3 m s-2.

()

10. A skater is pushing a snowmobile across an ice

rink. The skater pushed with a horizontal forceof 80 N and the snowmobile experiences a forceof friction of 30 N. The snowmobile has a

mass of 20 kg. What is the acceleration of thesnowmobile?A 4.0 m s-2 B 1.5 m s-2

C 2.5ms-2 D 0.25ms-2 ( )

1 l. An object of mass 5 kg is dropped from the top ofa building. Which of the following statements istrue? (Neglect air resistance)A It falls with a constant acceleration of 10 m s-2.

B It falls with a constant velocity of 10 m s-'.C It falls with a constant acceleration of 20 m s-2.

D It falls with a constant velocity of 5 m s-1.

()

12. When a horizontal force of 4 N is applied to a

wooden block of mass 2 kg on a horizontalsurface, the block moves with a constant velocity.If the force is increased to 12 N, what will be theacceleration of the block?A 2ms-2 B 4ms-2C 6ms-2 D 8ms-2 ( )

13. An empty lift of mass 200 kg is moving up with auniform velocity of 1.5 m s-r.

First 2 sA constantB increasing at

a constant rateC constant

D at an increasingconstant rate

After 2 szero

constant rate

increasing at aconstant rate

zeto()

velocity _

1.5 m s-l

weight

Taking gravitational acceleration to be 10 m s-2,

what is the resultant force on the lift in newtons?

A0 8200c300 D2000 ( )

14. A4.0 kg block of wood is pulled along a horizontalground from rest, and a force on 15 N is requiredto produce an acceleration of 2.0 m s-2.

What should the magnitude of the force be inorder to pull the block of wood at constant speed

of 5.0 m s-r on the same horizontal ground?

A 5N B 7Nc 8N D lON ( )

rE

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15. The diagram shows the velocity-time graph of an

object.

What can be said about the resultant force actingon the object?

A It is zero. B It is constant.C It is increasing. D It is decreasing.

()

16. An object is moving in a straight line at 6 m s-r

when a constant force acts on it from the oppositedirection. If the force is held constant for a longtime, how long will the object move?A It will stop moving immediately when the

force is applied.B It will decelerate and then stop.

C It will decelerate, stop and then move in theopposite direction with a constant speed.

D It will decelerate, stop and then movein the opposite direction with a constantacceleration. ( )

17. When a block of mass 1.2 kg is pushed with a

horizontal force of 3 N along a horizontal flatsurface, it moves with a constant speed of I .0 m s-r.

How will the block move when it is pushed alongthe same surface with a horizontal force of 9 N?A At a constant speed of 5 m s-r

B At a constant speed of 6 m s-l

C At a constant acceleration of 5 m s-2

D At a constant acceleration of 6 m s-2

()

18. A metal rod is used to raise a large stone as shownbelow.

metal

At which position A, B, C, or D should a force be

applied on the rod so that the large stone can be

moved most easily? ( )

19. A uniform metre rule is balanced at its centre ofmass as shown in the diagram. When masses Xand Y are hung from positions indicated, the ruleremains in equilibrium.

0 20 50 90 100 cm

I

What is the tension Zin the string?A 1.5N B 3.ONC 3.5N D 5.ON

g EWhat can be concluded about the masses X andY?

A X and Y have the same mass.

B X and Y have the same density.C The ratio of the mass of X to that of Y is 4:3.D The ratio of the mass of X to that of Y is 3:4.

()

20. A uniform metre rule of weight 3.5 N is hungfrom a piece of string tied at the 65 cm mark. Themetre rule is balanced by hanging a mass, ,fl, atthe end of it.

3.5 N

T

cB

21. The diagram below shows a light rod under theaction of three vertical forces. The points P, Q, R,S and T are equally spaced out along the rod. Atwhich point must an upward vertical force of 2 Nbe applied to hold the rod in equilibrium?

1N

BR

()

()PDASCA

large stone

stone

D

rod2N

1N

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22. Which of the following is in neutral equilibrium?A A heavy weight suspended on a stringB Acone resting on a slant edge

C A table lamp with a heavy base

D Apencil balanced on its sharp tip ( )

23. A 0.2 kg mass is thrown vertically upwards so

that it reaches a height of 3 m above the point ofprojection. What is its kinetic energy when it is2 m above the point of projection?

26. A girl weighing 400 N runs up a flight of stairsof vertical height 5 m in 4 s. What is her gain inpotential energy and the power developed?

Gain in potential PowerenergJi developed

A 1600J 400WB 2000J 500wc 16000 I 4000wD 20000J s0o0w ( )

21. A girl weighing 500 N runs up a flight of steps

6 m in height in 3 s. What average power is the

girl developing?A 250W B 1000Wc r500w D 2000w ( )

28. A 60-watt light bulb turns half the electricalenergy it uses into thermal energy. How much

thermal energy does it give out in 10 seconds?

A 6OJ B 6OOJ

c 300J D 0J ( )

29. When a car accelerates up a mountain, it

A gains both kinetic energy and potential energyB gains kinetic energy but loses potential energy

C gains potential energy but loses kinetic energyD loses both kinetic and potential energy

()

30. 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-2)

Fat man Thin manA 900w 400wB 1800w 800wc 9000w 4000wD l80w 80w ( )

31. A toy car K moving with a speed of 30 m s I has

kinetic energy of 900 J. Another identical toy carL moves with a speed of 15 m s '. What is thekinetic energy of the toy car L?A 225J B 45OJc 900J D 1800J ( )

A 140Wc 80w

B l00wD 60W

oI 1m

point of pOection

3m

BD4

AC

10JJ

6J2J ()

24. A block slides down from rest at point A.Assuming that 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

B

A 7.7 m s-l

B 8.5 m s-'C 9.0 m s-r

D cannot be determined

25. A man of mass 60 kg takes I minute to run upstairs from X to Y as shown in the diagram below.The man has a working power of

A

8m

m

()

()

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32. 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 l0 m below point X. (Take

I = l0 ms-2)

I'10 m

Find the speed of the ball at point Y.A 10.0 m s-' B l4.l m s-l

C l0Oms-r D l4lms-l

37 . A heavy car has high inertia. How difficult is it tostart it moving and to stop it moving?

To start To stopA easy easy

B difficult easy

C difficult difficultD easy difficult ( )

38. A rocket is launched from Earth into orbit. Whathappens to the mass and weight of the satellite?

Mass WeightA increases stays constantB decreases decreases

C stays constant decreases

D increases decreases ( )

39. When a force is applied to a body, several effectsare possible. Which of the following cannotoccur?A The body moves faster.

B The shape ofthe body is changed.C The body slows down.D The mass of the body increases. ( )

40. The diagram shows a uniform beam pivoted at

its centre, R. Forces, of 5 N and 6 N, act in thedirections shown.

o R S

0.2 m 0.2 m

5N 6N

What is the magnitude and position of the force

which will prevent the beam from turning?Magnitude Position

A I newton S

B l newton R

C llnewtons P

D 1l newtons a ( )

33. 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 144 m s-' B 72ms4C l2ms-' D 6.5ms-' ( )

34. The diagram below shows a box pulled by a

5 N force. A frictional force of 3 N acts in the

opposite direction. What is the work done by thebox against friction if it moves 5 m?

forcefrictionalforce of 3 N

of5

()

()0A

CJ

15JB 10JD 20t

35. An object of mass 0.3 kg moving at a constant

speed has a 21.6 J of kinetic energy. At what

speed is it moving?A l44ms-r B 72ms-lC 12ms-r D 6.5ms-r ( )

36. The total weight of an airship is 2000 N. The

airship rises at a constant speed of 8 m/s. Whatis the resultant force acting on the balloon duringthis ascent?

AONB 6OON

C IOOND 2000N ( )

I

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41. The diagrams below show a V-shaped objectplaced in different orientations. In which positiondoes the object exert the maximum pressure onthe surface it is resting?

A B

D

44. A lady weighs 500 N and wears stiletto shoes.The area of the stiletto heel is 0.0001 m2.

The pressure the lady exerts by standing on onestiletto heel isA 200 N m-2 B 5000 N mrC 500000Nmr D 5000000Nmr

()

45. The pressure exerted by a water jet of area 5 mm2

is 20 MPa (20 000 000 Pa).

5 mm2

water jet

What is the force exerted on the ground if the jetof water strikes the ground vertically?A lxl02N B lxl05NC lxl06N D lx108N ( )

C

()

42. A rectangular block weighing 192 N has

dimensions 8 cm x 6 cm x 2 cm. What is the

maximum pressure it can exert when it is placedon a horizontal surface?A 4 x 10aPa

B 72xl0aPaC 16 x 10aPa

D20x10aPa ( )

43. Two solid cubes are made from the same material.One cube has sides that are two times as long as

the other.

i,t! ,,t!,',"

v;'t""'

.I

When placed on one side, the small cube exerts apressure P on the ground. If the area of each faceof the small cube is z{, determine the pressureexerted by the large cube standing on one of itsfaces.

A2PC8P

B4PD I3P ()

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Short-structured Qrestions

1. A stationary trolley of mass 1.0 kg is free to move along a frictionless horizontal surface. For the

flrst 10 s, a steady horizontal force of 0.5 N is applied to the trolley.

(a) What happens to the speed of the trolley? tll

(b) What is the speed of the trolley after 10 s? t2l

(c) For the next 20 s, the applied force is removed. What happens to the speed of the trolley? [1]

(d) A new force, half the size of the first force and in the same direction, is applied for 30 s.

What happens to the speed of the trolley now? tll

(e) On the axes below, sketch the graph to illustrate the motion of the trolley during the 60 s. [2]

Speed(m s-1)

Time (s)

Z. The diagram below shows a block of mass 4 kg resting on a table. A force of 5 N is applied to the

block pulling it to the right, which gives it an acceleration of I m s-2.

4kg[-l- 5N

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(a) What is the resultant force acting on the block? tlI

(b) Explain why the resultant force is less than the applied force. tll

(c) If the applied force acts on the block for 5 s, what will the speed of the block be? tll

(d) After 5 s, the applied force is removed. What will the new acceleration be? tll

(e) How far has the block moved for the whole motion? I2l

3. Acarof mass 500 kg is being towed along a straightroad so that its velocity changes uniformly froml0 m s-t to 20 m s-r in a time of 8 seconds. During this time, the frictional resistance is constantand equals to 500 N.

(a) Calculate the acceleration of the car. t2)Danya

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(b) What is the resultant force on the car during the 8 seconds? tzl

(c) What is the force exerted on the car by the towing vehicle? t2)

(d) When the speed of 20 m s-r is reached, the towing force is reduced so that the car now moves

with constant velocity.

(i) What is the resultant force on the car now? tll

(ii) What is the towing force? tll

4. The diagram below shows a car moving along a horizontal road'

resistance

Friction

Forwarddriving force

The car has a mass of 1000 kg. At one point in its motion, when the combined forces of air resistance

and friction acting backward are 500 N, its acceleration is 1.6 m s-2'

Air

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(a) Calculate the forward driving force required to accelerate the car. t3l

(b) With the engine working at constant full power, the car's acceleration decreases as it goes

faster. Explain why this is so. 12)

5. A metal box, attached to a small parachute, is dropped from a helicopter.

(a) Explain, in terms of forces acting on the metal box, why its velocity increased immediately

afterbeing dropped. tll

(b) The total force opposing motion of the box and parachute at a particular instance during its

fall is 30 N. The combined mass of the box and parachute is 5.0 kg.

(i) Calculate the resultant downward force on the box and parachute.

(Take g to be 10 m s-2) l2l

(ii) Briefly describe the motion of the box and parachute at this time. t1lDanya

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(c) At the end of this fall, the parachute is caught on a tall tree. The box is then cut loose and fallsfrom rest to the ground. The duration of fall is 2.4 s. Calculate

(i) the velocity with which the box strikes the ground, t2)

(ii) the distance fallen, (Take g to be 10 m s-2) t2)

(iii) the average velocity during its fall. t2)

6. [n a performance test, a car of mass 800 kg is first accelerated to 20 m s-r on a dry straight road.After travelling at this speed for a while, the brakes are applied. The speed{ime graph of the motionof the car is shown below.

Speed (m s-1)

20

Time (s)

5.0 10.0 12.5 1 5.0

(a) What is the resultant force acting on the car in the time interval

(i) t=0to5.0s, tzl

0

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(ii) r = 5.0 s to 10.0 s? t1l

(b) Find the total distance travelled by the car, t3l

(c) Now the car is loaded so that its total mass is 1600 kg and is accelerated to 20 m s-r fromrest. If the net force acting on the car is unchanged, find the time taken for it to accelerate to20 m s-'. I2l

7. A block of mass 4.0 kg was placed on a rough, horizontal surface where the force of friction was

2.0 N. A man pulled the block with a force of 14 N for 2.0 s.

14N

(a) On the diagram above, draw an arrow to indicate the force of friction.

(b) What was the resultant, or effective, force causing the block to accelerate?

t1l

ttl

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(c) What was the acceleration of the block while the man was pulling? t2l

(d) What was the velocity of the block at the moment when the man let go? tzl

(e) What happened after 2.0 s? tll

8. A car of mass 1500 kg is moving along the road at constant velocity. The forward driving force is500 N and the frictional force isl

500 N

-15 000 N

(a) What is the magnitude of the frictional forcef t1l

(b) If the speed of the car is 25 m s-r, calculate the work done by the forward driving force in20 s. t2l

+ f

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(c) If the forward driving force now increases to 800 N, calculate the acceleration of the car. [2]

(d) How much further does the car travel in the next 10 s? l2l

9. A non-uniform plank weighing 300 N is set up as shown.The spring balance reads 160 N when a small boystands at A and 760 N when he stands at B.

5m

A B

2m 8m

(a) State what is meant by the moment of a force about a point. tll

ng

N

(b) Find the weight of the boy t3l

(c) How far is the centre of gravity of the plank from point A? tzl

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10. When large buildings are being erected, particularly onsoft ground, piles are driven into the ground to providea firm base. The diagram shows a pile hammer in operation.

The pile hammer has a mass of 2500 kg.

(a) Calculate the loss of gravitational potential energywhen the hammer falls 1.80 m to hit the pile. t21

Pile hammer

(b) What is the speed at which the hammer hits the pile? I2l

(c) The total mechanical energy of the pile and the hammer just after impact is 27 000 J.

(i) How much energy is lost? tll

tll

(d) The pile is knocked into the ground a distance of 10.0 m. Calculate the average resistance forceexerted on the pile while it is moving down. 12)

Pile

(ii) What has happened to the lost energy?

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11. (a) State a formula that relates pressure to force and area. tll

(b) The arm of a record player exerts a force of 0.01 N on a record. If the diameter of the stylus(contact point) is 0.0013 cm, estimate the pressure on the record groove. l2l

(c) When you drive or descend from a mountain, your ears may 'pop', which means that the

pressure behind the eardrum is being equalized to that outside.If this did not happen, what would be the approximate force on an eardrum of area 0.50 cm2

if a change in altitude of 1000 m takes place? t3l(Take density of air as 1.29 kg m-'and the acceleration of free fall as l0 m s-2)

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