Force And Laws Of Motion
Class 9th Science NCERT Exemplar Solution
- Which of the following statement is not correct for an object moving along a straight path…
- According to the third law of motion, action and reactionA. Always act on the same body B.…
- A goalkeeper in a game of football pulls his hands backwards after holding the ball shot…
- The inertia of an object tends to cause the objectA. To increase its speed B. To decrease…
- A passenger in a moving train tosses a coin which falls behind him. It means that motion…
- An object of mass 2 kg is sliding with a constant velocity of 4 ms-1 on a frictionless…
- Rocket works on the principle of conservation ofA. Mass B. Energy C. Momentum D. Velocity…
- A water tanker filled up to two - third of its height is moving with a uniform speed. On…
- There are three solids made up of aluminium, steel and wood, of the same shape and same…
- Two balls of the same size but of different materials, rubber and iron are kept on the…
- Two identical bullets are fired one by a light rifle and another by a heavy rifle with the…
- A horse continues to apply a force in order to move a cart with a constant speed. Explain…
- Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed…
- Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in…
- A truck of mass M is moved under a force F. If the truck is then loaded with an object…
- Two friends on roller-skates are standing 5 m apart facing each other. One of them throws…
- Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied.…
Multiple Choice Questions
Question 1.Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?
A. Its speed keeps changing
B. Its velocity always changes
C. It always goes away from the earth
D. A force is always acting on it
Answer:
Since, the object will be moving around the centre of the earth and being on the surface of the earth. So, in no ways it would go away from the earth.
Question 2.
According to the third law of motion, action and reaction
A. Always act on the same body
B. Always act on different bodies in opposite directions
C. Have same magnitude and directions
D. Act on either body at normal to each other
Answer:
Since, Newton’s third law states that for every action there is an equal and opposite reaction which means that in every interaction there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force of the second object.
Question 3.
A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to
A. Exert larger force on the ball
B. Reduce the force exerted by the ball on hands
C. Increase the rate of change of momentum
D. Decrease the rate of change of momentum
Answer:
A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal which increases the time span for the momentum to be reduced to zero which hence reduces the force.
Question 4.
The inertia of an object tends to cause the object
A. To increase its speed
B. To decrease its speed
C. To resist any change in its state of motion
D. To decelerate due to friction
Answer:
Since, inertia is the tendency of an object to do nothing or to remain unchanged in its state of motion or rest.
Thus, the inertia of an object tends to cause the object to resist any change in its state of motion.
Question 5.
A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is
A. Accelerated
B. Uniform
C. Retarded
D. Along circular tracks
Answer:
As it is only possible when the train is accelerating, the coin exist the inertia of motion in the forward direction and hence when it is tossed it falls backward.
Therefore, a passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is accelerated.
Question 6.
An object of mass 2 kg is sliding with a constant velocity of 4 ms–1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
A. 32 N
B. 0 N
C. 2 N
D. 8 N
Answer:
As it exist the inertia of motion and according to first law an object continues to be in its state of rest or motion unless an external force is applied to it.
Question 7.
Rocket works on the principle of conservation of
A. Mass
B. Energy
C. Momentum
D. Velocity
Answer:
According to the law of conservation of momentum which states that for two objects colliding in an isolated system the total momentum before and after the collision is equal.
Question 8.
A water tanker filled up to two - third of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would
A. Move backward
B. Move forward
C. Be unaffected
D. Rise upwards
Answer:
Since, the water exist the inertia of motion in the forward direction so when the brakes are applied the inertia of water resist the brakes applied. Hence, it moves forward.
Short Answer Type
Question 1.There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?
Answer:
Inertia depends upon mass of an object. Since, steel has greatest density and greatest mass therefore, it has the highest inertia. It is given that all have the same volume therefore, the object with large volume will have more mass. We know that the density of the steel is greatest of all thus steel would have higher energy.
Question 2.
Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.
Answer:
Yes. Both the balls will start rolling due to inertia of motion and they roll in the direction in which the train is moving. Due to the application of the brakes, the train comes to rest but due to inertia the ball tries to remain in motion, therefore, they continue to roll. The two balls will not roll with the same speed since, the masses of the ball are not same, therefore, the inertial force are also not same on both the balls thus the ball will move with difference speed.
Question 3.
Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why?
Answer:
As both the bullets are said to be identical and are fired with the same force, therefore, as per Newton’s third law of motion, every action has an equal and opposite reaction; same force will be applied on both the rifles. As the same amount force is applied on both the rifles, the lighter rifle will move more quickly in the backward direction causing serious injury to the shoulder.
Question 4.
A horse continues to apply a force in order to move a cart with a constant speed. Explain why?
Answer:
A horse continues to apply a force to move a cart with a constant speed so as to overcome the force of friction between the cart and the ground. Once the cart start moving, the force of friction comes into action. It starts working on the wheels of the cart in the direction opposite to the direction of the motion of the cart. So, the horse has to apply a continuous force in the forward direction to maintain constant speed.
Question 5.
Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentum of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?
Answer:
The law of conservation of momentum is applicable to isolated system. An isolated system is a system that is free from the influence of a net external force. In the given example, the change in velocity is due to the gravitational force of earth. This is an external force. Therefore, it is not an example of conservation of momentum.
Question 6.
Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in Fig. 9.1. Calculate the acceleration and frictional force of the floor on the ball.
Answer:
Given, m = 50g
F =?
We know, Force = mass × acceleration
= 
* 10 = 0.5 N
The velocity of the ball zero time is 80ms-1. It decelerates due to the friction of the floor with itself and comes to rest after 8 seconds.
a = 
= 
= -10m/s2
The negative sign indicates that the frictional force exerted opposes the motion of the ball. Now, using Newton’s relation,
F = m× a
= × -10
= -0.5 N
Question 7.
A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?
Answer:
Given, The initial mass m1 = M
Initial force, f1 = F
Given that, new mass m2 = M + M = 2M and new force f2 = 
Now, F = m×a
a = 
Initial acceleration (a1) = 
New acceleration (a2) = 
Change in acceleration = 
= 
= 4
Therefore, = 4
= a2
Thus, the new acceleration will be one–fourth of the old acceleration.
Question 8.
Two friends on roller-skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it. How will this activity affect the position of the two? Explain your answer.
Answer:
The initial momentum of both of them is zero but when one friend a ball of 2kg to other Newton’s third law of motion comes into action. Therefore, when one friend throws the ball to other he exerts a force on the ball and receives an equal and opposite force. In order to conserve the momentum, the one who throws the ball will move backward changing his initial position.
Now the other friend catches the ball, Law of conservation of momentum comes into action. In this case the momentum of the ball before catching and after catching will be equal. Therefore, the second will experience a net force after catching the ball and therefore will move backwards in the direction of the force. Thus, the separation between them will increase.
Question 9.
Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.
Answer:
The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkle, an equal and opposite reaction force comes into play so the sprinkler starts rotating.
Long Answer Type
Question 1.Using second law of motion, derive the relation between force and acceleration. A bullet of 10 g strikes a sand-bag at a speed of 103 ms-1 and gets embedded after travelling 5 cm. Calculate:
(i) The resistive force exerted by the sand on the bullet
(ii) The time taken by the bullet to come to rest.
Answer:
(i) Given,
m = 10g = 
kg
u = 103 m/s
v = 0, s = 5cm = 
m
v2 – u2 = 2as
0 – (103)2 = 2 × a ×
-(103) × (10)3 = 
a = (-1000 × 
) × 100
= -107 ms-2
F = ma
= 1/100x - 107
= -105 N
(ii) v = u +at
0 = (103) – 107 × t
t = 
t = 10-4s
Question 2.
Derive the unit of force using the second law of motion. A force of 5N produces an acceleration of 8 m s–2 on a mass m1 and an acceleration of 24 m s–2 on a mass m2. What acceleration would the same force provide if both the masses are tied together?
Answer:
We know,
Force = mass× acceleration
Unit of mass = kg
Unit of acceleration = m/s2
Therefore, unit of force = kg m/s2
1 kg m/s2 = 1 N
Given, F = 5 N, mass = m1
Acceleration, a = 8 m/s2
F = m × a
m1 = 
kg
Similarly, given F = 5N
Mass = m2
a = 24 m/s2
5 = m2 × 24 m/s2
m2 = 
kg
If both the masses are tie together,
Total mass = 
kg
Now, acceleration for mass 
kg
F = 5 N
F = m × a
a = 
= 6 m/s2
Question 3.
What is momentum? Write its SI unit. Interpret force in terms of momentum. Represent the following graphically
(a) Momentum versus velocity when mass is fixed.
(b) Momentum versus mass when velocity is constant.
Answer:
Momentum is the product of mass and velocity of an object. It is a vector quantity.
The SI unit of momentum is kg m/s.
The force acting on an object equals the rate of change of momentum of the object.
Force = rate of change in momentum
mv = constant
The change in momentum formula is given as:
p = mv
Therefore, when mass m is fixed, p is proportional to v.
Similarly, when v is constant, p is proportional to m.
Therefore, 
and 
graph are both straight lines.
