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ASSIGNMENT ON FORCES 1.

A powered spaceship is moving directly away from a planet as shown below. P S p a c e s h ip

p la n e t

At point P the motors of the spaceship are switched off but the spaceship remains under the influence of the planet. Which one of the following graphs best represents the variation with time t of the velocity v of the spaceship after it passes point P? A .

B.

v

0

C.

0

0

t

D .

v

0

v

0

t

v

0 t

0

0

t

(1)

2.

For an object to be in translational equilibrium A.

it must be at rest.

B.

it must be moving with a constant acceleration.

C.

no external force must be acting on it.

D.

the net force acting on it must be zero. (1)

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3.

A bird of weight W lands at the midpoint of a horizontal wire stretched between two poles. The magnitude of the force exerted by each pole on the wire is F. F

F

W

The bird will be in equilibrium if A.

2F > W.

B.

2F = W.

C.

2F < W.

D.

F = W. (1)

4.

A weight is suspended from a spring. The variation with weight of the length of the spring is shown below. le n g th / c m

16

14

12

10

0

2

4

6

8 w e ig h t / N

2


What is the value of the spring constant (force constant) of the spring? A.

0.4 N cm–1

B.

0.5 N cm–1

C.

2.0 N cm–1

D.

2.5 N cm–1 (1)

5.

A mass is suspended from the roof of a lift (elevator) by means of a spring balance, as illustrated below.

lift (e le v a to r)

m ass

The lift (elevator) is moving upwards and the readings of the spring balance are noted as follows. Accelerating:

Ra

Constant speed:

Rc

Slowing down:

Rs

3


Which one of the following is a correct relationship between the readings? A.

Ra > Rc

B.

Ra = Rs

C.

Rc = Rs

D.

Rc < Rs (1)

6.

A friction force f is acting on a block of weight w sliding down an incline at a constant speed. The force N is the normal reaction of the incline on the block. Which of the following free-body diagrams best represents the forces acting on the block? A .

B.

f

N

N

f

w w

C .

f

N

D .

N

f

w

w

(1)

4


7.

A ball of weight W is dropped on to the pan of a top pan weighing balance and rebounds off the pan.

pan

0 0 .0 0 At the instant that the ball has zero velocity when in contact with the pan, the scale will read A.

zero.

B.

a value less than W but greater than zero.

C.

W.

D.

a value greater than W. (1)

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8.

The engine of a rocket ejects gas at high speed, as shown below. ro c k e t

h ig h s p e e d gas

d ire c tio n o f m o tio n o f ro c k e t

The rocket accelerates forwards because A.

the momentum of the gas is equal but opposite in direction to the momentum of the rocket.

B.

the gas pushes on the air at the back of the rocket.

C.

the change in momentum of the gas gives rise to a force on the rocket.

D.

the ejected gas creates a region of high pressure behind the rocket. (1)

9.

Which one of the following is the condition necessary for an object to be in translational equilibrium? A.

The lines of action of all the forces acting on the object must pass through a single point.

B.

Every force must be balanced by another force that is equal in magnitude but opposite in direction.

C.

The resultant of all the forces acting on the object in any direction must be zero.

D.

The total upward force on the object must be equal to the total downward force. (1)

6


10.

A frictionless trolley of mass m moves down a slope with a constant acceleration a. A second similar frictionless trolley has mass 2m. The acceleration of the second trolley as it moves down the slope is

A.

1 a. 2

B.

a.

C.

2a.

D.

4a. (1)

11.

A block rests on a rough surface. Two forces P and Q act on the block, parallel to the surface. A friction force F between the block and the surface keeps the block in equilibrium. Which vector diagram best represents the three forces? A .

B. Q

P

P F F Q

C.

D . Q P

P F

F Q (1)

7


12.

A body starting from rest moves along a straight-line under the action of a constant force. After travelling a distance d the speed of the body is v. in itia l p o s itio n v

d

d The speed of the body when it has travelled a distance 2 from its initial position is

A.

v . 4

B.

v . 2 v

C.

2

.

v

D.

2 2

. (1)

8


13.

A body of weight 2W hangs vertically from a string attached to a body of weight W. Weight W is released and both bodies fall vertically. W

2W

Air resistance may be neglected. What is the tension in the string during the fall? A.

Zero

B.

W

C.

2W

D.

3W (1)

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14.

A uniform metal bar XY of weight W is hung from a horizontal support at point P by two wires of negligible mass. P T

T

X

Y

W Each wire makes an angle θ with the vertical. Which of the following is equal to the tension T in one of the wires?

A.

W cos θ

B.

W 2 cos θ

C.

W sin θ

D.

W 2 sin θ (1)

10


15.

A picture is supported vertically by a wire that is looped over a horizontal light peg P. There is no friction between the wire and the peg. P peg w ire Y

X

p ic tu re

The mass of the picture is uniformly distributed and PX = PY. Which of the following best represents the free body diagram of the forces acting on the peg?

A .

C .

B .

D .

(1)

11


16.

A small steel ball falls from rest through a distance of 3 m. When calculating the time of fall, air resistance can be ignored because A.

air is less dense than steel.

B.

air resistance increases with the speed of the ball.

C.

the air is not moving.

D.

air resistance is much less than the weight of the ball. (1)

17.

The graph below shows the variation with load of the length of a spring. le n g th

L Z Y

X 0

0

W

lo a d

For a load W the length of the spring is L. The spring constant of the spring is given by A.

the gradient of the graph.

B.

1 gradient of the graph .

C.

W L .

D.

L W . (1)

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18.

A fireman is holding a hosepipe so that water leaves the pipe horizontally. The hosepipe has a constant cross-sectional area. The magnitude of the force that the fireman exerts to hold the hosepipe stationary is F. The volume of water delivered by the hose per second doubles, the force that the fireman must now exert is A.

2F .

B.

2F.

C.

4F.

D.

8F. (1)

19.

Mandy stands on a weighing scale inside a lift (elevator) that accelerates vertically upwards as shown in the diagram below. The forces on Mandy are her weight W and the reaction force from the scale R.

The reading of the scale is A.

R + W.

B.

W.

C.

R.

D.

R â&#x20AC;&#x201C; W. (1)

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20.

An object of mass M is suspended from a spring. The extension of the spring is e. The same object is suspended from an identical spring on the Moon where the acceleration of free fall is less than that on Earth. Which of the following is correct? Mass of the object on Moon Extension of spring on Moon A.

M

e

B.

less than M

less than e

C.

M

less than e

D.

less than M

e (1)

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ASSIGNMENT ON FORCES