# Assertion and Reason Questions for Class 9 Science Chapter 9 Force and Laws of Motion

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Directions: In each of the following questions, a statement of Assertion is given and a corresponding statement of Reason is given just below it. Of the statements, given below, mark the correct answer as:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.

Q.1. Assertion : If the net external force on the body is zero, then its acceleration is zero.
Reason : Acceleration does not depend on force.

According to Newton’s second law,
force = acceleration x mass
i.e., if net external force on the body is zero (F = 0), then, the acceleration of a body is also zero.

Q.2. Assertion : A rocket works on the principle of conservation of linear momentum.
Reason : For two bodies system when there is a change in momentum of one body, the same change occurs in the momentum of the second body but in the opposite direction.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
As the fuel in rocket undergoes combustion, the gases so produced leave the body of the rocket with large velocity and give upthrust to the rocket. If we assume that the fuel is burnt at a constant rate, then the rate of change of momentum of the rocket will be constant. As more and more fuel gets burnt, the mass of the rocket goes on decreasing and it leads to increase of the velocity of rocket more and more rapidly.

Q.3. Assertion : A quick collision between two bodies is more violent than a slow collision, even when the
initial and the final velocities are identical.
Reason : Because the rate of change of momentum which determines the force is greater in the first case.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Q.4. Assertion : When a firefly hits a bus, each of them exerts the same force.
Reason : Firefly has more mass as compared to the windshield.

Q.5. Assertion : Linear momentum is conserved in both elastic and inelastic collisions.
Reason : Total energy is conserved in all collisions.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.
Both linear momentum and kinetic energy are conserved in elastic collisions. Linear momentum is conserved in inelastic collisions but kinetic energy is not conserved. The total energy is conserved in all collisions.

Q.6. Assertion : Force required to accelerate a mass in two perpendicular directions is never same.
Reason : The presence of g will not influence the acceleration.

Q.7. Assertion : A quick collision between two bodies is more violent than a slow collision, even when the
initial and the final velocities are identical.
Reason : The rate of change of momentum determines the force.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
In quick collision, time of impact is small which increases the impact of force.

Q.8. Assertion : While walking on ice, one should take small steps to avoid slipping.
Reason : This is because smaller steps ensure smaller friction.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Q.9. Assertion : From Newton’s second law of motion, impulse is equal to change in momentum.
Reason : Impulse and momentum have different SI units.

As impulse is equal to change in momentum. Therefore, impulse and momentum have same SI units.

Q.10. Assertion : Newton’s third law applies to all types of forces. e.g. gravitational, electric or magnetic forces etc.
Reason : Newton’s third law of motion is applicable only when bodies are in motion.

According to third law of motion it is impossible to have a single force out of mutual interaction between two bodies, whether they are moving or at rest. While, Newton’s third law is applicable for all types of forces.

Q.11. Assertion : A body is momentarily at rest when it reverses the direction.
Reason : A body cannot have acceleration if its velocity is zero at a given instant of time.

Q.12. Assertion : When a bullet is fired from a gun, there is a forward force on the bullet and recoil of gun.
Reason : Every action has an equal and opposite reaction.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Q.13. Assertion : When astronauts throw something in space, that object would continue moving in the same
direction and with the same speed.
Reason : The acceleration of an object produced by a net applied force is directly related to the magnitude
of the force, and inversely related to the mass of the object.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Q.14. Assertion : A large number of concurrent forces acting at the same point of the object, then the object will be in equilibrium, if sum of all the forces is equal to zero.
Reason : Equilibrium of a particle in mechanics refers to the situation when the net external force on the particle is non-zero.

Q.15. Assertion : When we sit on a chair, our body exerts a force downward and that chair needs to exert an equal force upward or the chair will collapse.
Reason : The third law says that for every action there is an equal and opposite reaction.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Q.16. Assertion : A table cloth cannot be pulled from a table without dislodging the dishes.
Reason : Newton’s second law of motion gives definition of inertia.

Q.17. Assertion : Force exerted by the ground on the man moves him forward.
Reason : It is a reactional force.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Q.18. Assertion : Mass is a measure of inertia of the body in linear motion.
Reason : Greater the mass, greater is the force required to change its state of rest or motion.

Answer Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
According to Newton’s second law of motion, F =ma i.e., magnitude of the acceleration produced by a given force is inversely proportional to the mass of the body. Higher is the mass of the body, lesser will be the acceleration produced i.e., mass of the body is a measure of the opposition offered by the body to change a state, when the force is applied i.e., mass of a body is the measure of its inertia.

Q.19. Assertion : Change in momentum is impulse.
Reason : Impulse is the area between (F-t) graph and time axis.

Answer Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion.

Q.20. Assertion : A boy facing forward in a moving bus throws a ball straight up. At the same instant the bus begins to accelerate. The ball goes up and falls in front of the boy.
Reason : As the ball rises, velocity remains constant.