# Assertion and Reason Questions for Class 11 Physics Chapter 5 Laws of Motion

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#### Assertion and Reason Questions for Class 11 Physics Chapter 5 Laws of Motion

Directions : Each of these questions contain two statements, Assertion and Reason. Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Q.1. Assertion : On a rainy day, it is difficult to drive a car or bus at high speed.
Reason : The value of coefficient of friction is lowered due to wetting of the surface.

Answer Answer: (a) On a rainy day, the roads are wet. Wetting of roads lowers the coefficient of friction between the types and the road. Therefore, grip on a road of car reduces and thus chances of skidding increases.

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

Q.3. Assertion : An object can move with constant velocity if no net force acts on it.
Reason : No net force is needed to move an object with constant velocity.

Q.4. Assertion : If the net external force on the body is zero, then its acceleration is zero.
Reason : Acceleration does not depend on force.

Q.5. Assertion : For the motion of electron around nucleus, Newton’s second law is used.
Reason : Newton’s second law can be used for motion of any object.

Answer Answer: (c) Newton’s second law can not be used for any object.

Q.6. 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 of uniform motion.

Answer Answer: According to Newton’s second law of motion a = F/m 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.7. Assertion : Impulse of force and momentum are same physical quantities.
Reason : Both quantities have same unit.

Answer Answer: (d) Impulse and momentum are different quantities, but both has same unit (Nâ€“s).

Q.8. Assertion: Same force applied for the same time causes the same change in momentum for different bodies
Reason: The total momentum of an isolated system of interacting bodies remains conserved.

Q.9. Assertion : A bullet is fired from a rifle. If the rifle recoils freely, the kinetic energy of rifle is more than that of the bullet.
Reason : In case of rifle bullet system, the law of conservation of momentum violates.

Q.10. Assertion: A cricketer moves his hands forward to catch a ball so as to catch it easily without hurting.
Reason: He tries to decrease the distance travelled by the ball so that it hurts less.

Answer Answer: (d) A cricket player moves his hands backward to increase the time interval for reducing the momentum of the ball to zero. Thus the ball does not hit him hard as force is directly proportional to change of momentum.

Q.11. Assertion : The two bodies of masses M and m (M > m) are allowed to fall from the same height if the air resistance for each be the same then both the bodies will reach the earth simultaneously.
Reason : For same air resistance, acceleration of both the bodies will be same.

Q.12. Assertion : A block placed on a table is at rest, because action force cancels the reaction force on the block.
Assertion : The net force on the block is zero.

Answer Answer: (d) The net force on the block is zero, but action cannot cancel the reaction because these two act on different bodies.

Q.13. Assertion : Frictional forces are conservating forces.
Reason : Potential energy can be associated with frictional forces.

Q.14. Assertion : A man and a block rest on smooth horizontal surface. The man holds a rope which is connected to block. The man cannot move on the horizontal surface.

Reason : A man standing at rest on smooth horizontal surface cannot start walking due to absence of friction (The man is only in contact with floor as shown).

Answer Answer: (d) The man can exert force on block by pulling the rope. The tension in rope will make the man move. Hence statement-1 is false.

Q.15. Assertion: Friction is a necessary evil
Reason: Though friction dissipates power, but without friction we cannot walk.

Answer Answer: (a) Friction causes wear & tear and loss of energy, so it is an evil but without friction walking. Stopping a vehicle etc. would not be possible. So it is necessary for us.

Q.16. Assertion: There is a stage when frictional force is not needed at all to provide the necessary centripetal force on a banked road.
Reason: On a banked road, due to its inclination the vehicle tends to remain inwards without any chances of skidding.

Answer Answer: (c) The assertion is true for a reason that when the car is driven at optimum speed. Then the normal reaction component is enough to provide the centripetal force.

Q.17. Assertion : A cyclist always bends inwards while negotiating a curve.
Reason : By bending, cyclist lowers his centre of gravity

Answer Answer: (c) The purpose of bending is to acquire centripetal force for circular motion. By doing so component of normal reaction will counter balance the centrifugal force.

Q.18. Assertion : Force is required to move a body uniformly along a circle.
Reason : When the motion is uniform, acceleration is zero.

Answer Answer: (b) When a body is moving in a circle, its speed remains same but velocity changes due to change in the direction of motion of body. According to first law of motion, force is required to change the state of a body. As in circular motion the direction of velocity of body is changing so the acceleration cannot be zero. But for a uniform motion acceleration is zero (for rectilinear motion).

Q.19. Assertion : Linear momentum of a body changes even when it is moving uniformly in a circle.
Reason : In uniform circular motion, velocity remains constant.

Answer Answer: (c) In uniform circular motion, the direction of motion changes, therefore velocity changes. As P = mv therefore momentum of a body also changes uniform circular motion.

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