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Revision Question Bank
Force and Laws of Motion
1.
Which one has greater inertia, a stone of mass 1 kg or a stone of mass 5 kg?
Sol.
Stone of mass 5 kg.
2. Why does a gun recoil?
Sol.
Since no force acts, so to conserve momentum, gun recoils.
3. What is the condition for conservation of momentum?
Sol.
Net external force acting on the mass should be zero.
4. Any luggage kept on the roof of cars or buses is always tied with a rope. Why?
Sol.
When break is applied, they tend to displace back and fall. So, it is advised to tie them
with a rope.
5. Why are we hurt more during a fall on a concrete structure than on a sandy track?
Sol.
In a concrete structure, our velocity/momentum is made zero in a very short time, while
in a sandy track, the track yields to our force and increases the time taken.
1
Since, F  , we feel more hurt in a concrete track,
t
6. Why is it dangerous to jump out of a moving bus?
Sol.
Inertia of motion will not permit you to stop immediately. Chances of having a fall exist.
So, it is dangerous.
7. In the following experimental set-up, a student gives the card a sharp, fast horizontal
flick with a finger.
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(a) what will happen to the coin?
(b) State reason for your answer.
Sol.
(a) The coin will fall in the glass.
(b) Both coin and card tend to remain at rest. On applying unbalanced force on card, the
coin falls vertically.
8. According to third law of motion, when we push an object, the object pushes back on us
with equal and opposite force. If the object is a massive truck parked along the roadside,
it will probably not move even if we apply force on it. A student justifies this by
answering that two equal and opposite forces cancel each other. Comment on this logic
and explain why the truck does not move.
Sol.
The logic is wrong as force of action and reaction act on two different bodies and do not
cancel each there.
The truck does not move because the force applied on it, is very small and is being
balanced by the force of friction acting on the truck due to road. The truck
is a massive one and has large inertia and to bring it to motion, we require an
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Chapter Test {Force and Laws of Motion}
M: Marks: 30
1.
M: Time: 40 Min.
Give reasons
[5]
(a) It is difficult to balance our body, when we accidentally step on a
peel of a banana.
(b) Pieces of bursting cracker fall in all possible directions.
(c) A glass pane of a window is shattered, when a flying pebble hits it.
(d) It is easier to stop a tennis ball than a cricket ball moving at the same speed.
(e) Javelin thrower is marked foul, if an athlete crosses over the line marked for the
throw. Athletes often fail to stop themselves before the line.
Sol.
(a) The reason is that one cannot exert action force effectively on the slippery peel of
banana in the backward direction. Hence, in response, the ground does not exert
sufficient reaction force in forward direction and hence we lose our balance.
(b) This can be explained on the law of conservation of momentum. When due to
explosion some pieces move in the same particular direction, then in order to conserve
momentum, the remaining pieces move in the opposite direction.
(c) The reason is that, the glass pane of the window is a hard solid. The flying pebble
suffers change in momentum in a very short time, so the force exerted by the glass
window on the pebble will be large. Consequently the glass pane of the window will
shatter.
(d) The reason is that mass of the cricket ball is more than that of a tennis ball.
Thus,
momentum is more in case of the cricket ball due to larger mass as compared to the
tennis ball. So, less force has to be applied in the case of the tennis ball to stop.
(e) The reason is that when athletes are running for the throw then due to inertia of
motion they often fail to stop themselves before the line.
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2.
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(a) How will you define unit of force using the second law of motion
[3]
(b) Calculate the mass of a body. When a force of 525 N produce an acceleration of
3.5m/s 2.
Sol.
(a) SI unit of force is 1 newton (1N).
According to second law of motion F = ma If mass of an object, m = 1 kg and a = 1 ms –2,
then F =1kg×lms–2 =1N
Hence, one newton force is the force, which when acting on an object of mass 1 kg
produces an acceleration of 1 ms–2 in it.
(b) Here, force, F = 525 N and acceleration, a = 3.5ms–2
 mass of body m =
3.
F 525

 150kg
a 35
(a) Name the property of bodies to resist a change in their velocity.
[5]
(b) What is the relationship between force and acceleration?
(c) What name is given to the product of mass and velocity of a body?
(d) Which physical quantity corresponds to the rate of change of momentum?
(e) Name the principle on which a rocket works.
Sol.
(a) The property of bodies to resist a change in their velocity is inertia.
(b) The relation between force and accelerations is
F= m × a
(c) The product of mass and velocity of body is momentum.
(d) The physical quantity that corresponds to the rate of change of momentum is force.
(e) The principle on which a rocket works is Newton's third law of motion.
4.
Answer the following questions
[2]
(a) When a body actually begins to move over the surface of another, which force comes
into play?
(b) What type of work is done by frictional force?
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Sol.
(a) Kinetic friction comes into play.
(b) Negative work is done by frictional force.
5.
A constant force acts on an object of mass 5 kg for a duration of 2 s. It increases the
velocity of object from 3 ms–1 to 7 ms–1 Find the magnitude of applying force.
Now if the force was applied for a duration of 5 s, what would be the final velocity of
object?
[3]
Sol.
(a) Here initial velocity, u = 0
Acceleration of stone, a = +g = + 9.8 ms–2
Time, t = 5s
 Distance covered by the stone = height of the hill
1
1
h = gt2 = × 9.8 × (5)2 = 122.5 m
2
2
Again final velocity of stone at instant of its just reaching the ground,
v = gt = 9.8 × 5 = 49 ms–1
(b) Here initial velocity, u = 0 Acceleration, a = + g = + 9.8 ms –2
Time, t = 3 s
Vertically downward distance covered
1
1
h = gt2 = ×9.8×(3)2 =44.1m
2
2
6.
Answer the following questions
[5]
(a) Difference between balanced force and unbalanced force.
(b) What is inertia.
(c) When a bike accelerates suddenly, the back seater moves backwards and when the
breaks are applies to the bike, the seater moves forward, why
Sol.
(a) Balanced Force If net force on a body is zero it is called balanced force. Balanced
forces do not cause a change in motion. They are equal in magnitude and opposite in
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direction. Therefore, the resultant of these forces will be zero.
For example, two persons pushing a box with the same force in opposite directions.
Unbalanced Force If net force on a body is non-zero it is called unbalanced force.
Forces whose resultant is not equal to zero are called unbalanced forces. For example,
an arm wrestling competition among a strong person and a weak one. The
resultant force will be in the direction of the force applied by the strong person.
(b) By Newton's first law, we know that a body at rest will remain at rest and a body in
motion will continue in a straight line unless it is compelled by an external force. This
tendency of a body is called inertia. So, we can say that 'Inertia is that property of a body
due to which it resists a change hi its state of rest or uniform motion.'
(c) When the bike accelerates suddenly, the back seater who was initially at rest
experiences a force of friction at the point of contact between him and the bike. His
inertia of rest tries to keep him at rest which results in a backward push. Thus, the back
seater moves backwards when the bike accelerates suddenly.
(d) When the brakes are applied, the inertia of motion of the back seater tries to keep
him in motion so he moves forward.
7.
State Newton's second law of motion and establish the relation F=ma.
[4]
Give reasons for the following
(i) If a boy jumps out of a boat, the boat moves backwards.
(ii) The passengers sitting in a bus fall backwards when the bus starts suddenly.
Sol.
(a) The force acting on a mass is directly proportional to the product of its mass and
acceleration, i.e., F = ma
This force is equal to the rate of change of momentum of the body.
i.e., force, F =
=
f  i
t
mv  mu m  v  u 

 ma
t
t
 F  ma
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(b) (i) This is according to Newton's third law of motion, the boy pushes the boat
backward and in turn the boy jumps forward.
(ii) This is due to inertia of rest, as passengers are initially hi the state of rest and tend
to remain in the same state.
8.
(i) Explain why is it difficult to walk on sand?
[3]
(ii) Why is the recoil of a heavy gun, on firing, not so strong as that of a light gun using
the same cartridge?
(iii) A constant force acts on an object of 5 kg for a period of 2 s. It increases the velocity
of an object from 3 m/s to 7 m/s. Find the magnitude of the applied force. Now if the
force was applied for a period of 5 s, what would be the final velocity of the object?
Sol.
(i) While walking on sand, the sand gets pressed down, impacting less reaction force on
the person.
(ii) The mass of the gun is higher. So, it coil's velocity is less.
(iii) Given, m = 5
kg, t1 = 2s,
u=3 m/s, v = 7 m/s, F =1 t2 =5s, v2 =?
 v u
F  m

 t1 
 7 3 
=5 
 =10N
 2 
v2 =u + at2 = 3+
10  5
 13m / s
5
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