  Question

Read each statement below carefully, and state, with reasons, if it is true or false; (a) During rolling, the force of friction acts in the same direction as the direction of motion of the CM of the body. (b) The instantaneous speed of the point of contact during rolling is zero. (c) The instantaneous acceleration of the point of contact during rolling is zero. (d) For perfect rolling motion, work done against friction is zero. (e) A wheel moving down a perfectly frictionless inclined plane will undergo slipping (not rolling) motion.

Solution

(a) The resisting force in the rolling motion is called as rolling friction. The rolling friction acts when a round object rolls on another surface. During rolling motion without slipping, the friction force acts in the same direction as the direction of motion of the centre of mass of the body to prevent it sliding in the backward direction Thus, the given statement is true. (b) A point on or off the body about which the body is assumed a purely rotational motion is called as instantaneous center of rotation. The rolling body makes contact with ground during rolling motion and contact point has zero velocity. Thus the instantaneous speed of contact point is zero. Thus, the given statemet is true. (c) The acceleration is the rate of change of the velocity with respect to time. The instantaneous speed of contact point has zero velocity. Thus, the instantaneous acceleration of the point of contact is not zero. Thus, the given statement is false. (d) The work done define as the product of force and displacement of the object. During perfect rolling, there is no relative motion at the point of contact. The displacement is zero. Thus, the work done against the friction is zero. Thus, the given statement is true. (e) Body rolls on inclined plane due to friction and tangential force which provide torque. When inclined plane is perfectly smooth, there is no friction and it will slip under the effect of its own weight. Thus the given statement is true. PhysicsPhysics Part-I - NCERTStandard XI

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