NCERT Exemplar Problems Class 9 Science – Force and Laws of Motion

Question 1:
Which of the following statement is not correct for an object moving along a straight path in 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:
(c) For an object moving along a straight path in an accelerated motion, it is not necessary that it always goes 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 direction
(d) act on either body at normal to each other
Answer:
(b) According to third law of motion, action and reaction always act on different bodies in opposite directions

Question 3:
A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper 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:
(d) The goalkeeper pulls his hands backwards after holding the ball to decrease the rate of change of momentum by increasing the time. By doing this, less force is exerted on his hands (v Force is directly proportional to the rate of change of momentum).

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:
(c) The inertia of an object tends to cause the object to resist any change in its state of rest or 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:
(a) If the coin falls behind the passenger that means the train is accelerated. When the coin is tossed it has same velocity as that of train but during the time it is in air its velocity becomes less than that of train (because the train is accelerated), so it falls behind the passenger.

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:
(b) Given, mass m = 2 kg, velocity v = 4 ms^-1
As the object is moving with a constant velocity i.e., 4 ms^-1 so the acceleration of the object is zero i.e.,
a = 0 and according to the property of inertia if there is no external force acting on the body, then body remains as it is i.e., if the body is at rest, remains and if it is in motion remains in motion.

Question 7:
Rocket works on the principle of conservation of
(a) mass (b) energy (c) momentum (d) velocity
Answer:
(c) Rocket works on the conservation of momentum. In a rocket, the fuel burns and produces gas at high temperature. These gases are ejected out of the rocket from a nozzle at the back side of the rocket. The ejecting gas exerts a forward force on the rocket which help in accelerating.
Through the mass of gases escaping per second is very small and their momentum is very large due to their tremendous velocity of escape. An equal and opposite momentum is imparted to the rocket which despite its large mass builds up a high velocity.

Question 8:

(a) move backward (b) move forward
(c) be unaffected (d) rise upwards
Answer:
(b) On the sudden application of brake, the tanker will come in the state of rest but the water remains in the state of motion, so the water will move forward.

Short Answer Type Questions

Question 9:
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:
As the mass is a measure of inertia the ball of same shape and size, having more mass than other balls will have highest inertia. Since , the steel has greatest density and greatest mass therefore it has highest inertia.

Question 10:
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:
When the train is stopped suddenly, then it comes in the state of rest but the balls remain in the state of motion. So, due to inertia of motion, the balls move in the forward direction.
As the balls are of same size but of different materials that means their mass will be different. So, both the balls will move with different speeds.

Question 11:
Two identical bullets are fired one by 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 identical and are fired with the same force. So, to reduce recail velocity, we use, heavy rifle so that its heavy mass compansate the large momentum. But for light rifle, recoil velocity will be large due to small mass thus light rifle will hurt more to the shoulder.

Question 12:
A horse continues to apply a force in order to move the cart with a constant speed. Explain why?
Answer:
When the cart starts moving, frictional force starts working on the wheel of cart in opposite to the motion. So, horse need to apply continuous force in forward direction to maintain constant speed.

Question 13:
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 attain the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?
Answer:
Momentum of a system remains conserved if no external force acts on the system. In the given example, there is gravitational force acting on the ball which is an external force, so it is not an example of conservation of momentum.

Question 14:
Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in figure.
Calculate the acceleration and frictional force of the floor on the ball.

Answer:

Question 15:
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:

Question 16:
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:
Separation between them will increase. Initially the momentum of both of them are zero as they are at rest. In order to conserve the momentum the one who throws the ball would move backward.
The second will experience a net force after catching the ball and therefore will move backward that is in the direction of the force.

Question 17:
Water sprinkler used for grass lawn begins to rotate as soon as the water is supplied. Explain the principle on which it works.
Answer:
Water sprinkler works on Newton’s third law of motion. When the water supplied, then force applies on the sprinkler due to which it rotates.

Long Answer Type Questions

Question 18:
Using second law of motion, derive the relation between force and acceleration. A bullet of 10 g strikes a sand-bag at 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:
If a body of mass (m), moving at velocity (u) accelerates uniformly at (a) for time T, so that its velocity changes to v, then

Question 19:
Derive the unit of force using the second law of motion. A force of 5 N produces an acceleration of 8 ms^-2 on a mass m₁ and an acceleration of 24 ms^-2 on a mass m_z. What acceleration would the same provide, if both the masses are tied together?
Answer:
We know that, SI unit of mass, m = 1 kg
SI unit of acceleration, a = 1 ms^-2
Now, from Newton’s second law, F = ma
SI unit of force, F = 1 kg x 1 ms^-2
= 1 kg-ms^-2
1 kg-ms-2 is known as 1 newton (N)
∴ 1 newton (N) = 1 kg-ms^-2
1 newton can be defined as, “the force is said to be 1 newton if it produces 1 ms^-2
acceleration in a body of 1 kg mass”.
Condition I Given, F1 = 5 N, a, = 8 ms^-2 and m, = mass of first body
F2 = 5 N; a2 = 24 ms^-2 and m2 = mass of second body

Question 20:
What is momentum? Write its SI unit. Interpret force in terms of momentum. Represent the following graphically
(i) momentum versus velocity when mass is fixed.
(ii) momentum versus mass when velocity is constant.
Answer:
Momentum The quantity of motion possessed by a moving body is known as momentum of the body.
Momentum of an object of mass m moving with a velocity v is the product of it.
It is denoted by p
p = mv
Its SI unit is kg-ms^-1.
As from Newton’s llnd law, rate of change of momentum is equal to force applied. So,