Assertion & Reason

Question 11 Mark

Assertion : The acceleration of a body down a rough inclined plane is greater than the acceleration due to gravity.
Reason : The body is able to slide on a inclined plane only when its acceleration is greater than acceleration due to gravity.

Answer

(d) Acceleration down a rough inclined plane $a=g(\sin \theta-\mu \cos \theta)$ and this is less than $g$.

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Question 21 Mark

Assertion : The value of dynamic friction is less than the limiting friction.
Reason : Once the motion has started, the inertia of rest has been overcome.

Answer

(a)

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Question 31 Mark

Assertion : Friction is a self adjusting force.
Reason : Friction does not depend upon mass of the body.

Answer

(d) Only static friction is a self adjusting force. This is because force of static friction is equal and opposite to applied force (so long as actual motion does not start). Frictional force $=\mu mg$ i.e. friction depends on mass.

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Question 41 Mark

Assertion : 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.

Answer

(d) The force acting on the body of mass $M$ are its weight $M g$ acting vertically downwards and air resistance $F$ acting vertically upward.$\therefore$ Acceleration of the body, $a=\frac{M g-F}{M}=g-\frac{F}{M}$Now, $M>m$, therefore, the body with larger mass will have greater acceleration and it will reach the ground first.

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Question 51 Mark

Assertion : Angle of repose is equal to angle of limiting friction.
Reason : When the body is just at the point of motion, the force of friction in this stage is called as limiting friction.

Answer

(b)

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Question 61 Mark

Assertion : Pulling a lawn roller is easier than pushing it.
Reason : Pushing increases the apparent weight and hence the force of friction.

Answer

(a) Suppose the roller is pushed as in figure (b). The force $F$ is resolved into two components, horizontal component $F$ which helps the roller to move forward, and the vertical component acting downwards adds to the weight. Thus weight is increased. But in the case of pull [fig (a)] the vertical component is opposite to its weight. Thus weight is reduced. So pulling is easier than pushing the lawn roller.

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Question 71 Mark

Assertion : When a bicycle is in motion, the force of friction exerted by the ground on the two wheels is always in forward direction.
Reason : The frictional force acts only when the bodies are in contact.

Answer

(e) When a bicycle is in motion, two cases may arise :(i) When the bicycle is being pedalled. In this case, the applied force has been communicated to rear wheel. Due to which the rear wheel pushes the earth backwards.

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Question 81 Mark

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

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

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JEE physics Assertion & Reason

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