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  4. Assertion The work done against force of friction in the case of a disc rolling without slipping down an inclined plane is zero. Reason When the disc rolls without slipping, friction is required because for rolling condition velocity of point of contact is zero.

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Q. Assertion The work done against force of friction in the case of a disc rolling without slipping down an inclined plane is zero.
Reason When the disc rolls without slipping, friction is required because for rolling condition velocity of point of contact is zero.

System of Particles and Rotational Motion

Solution:

The work done on a body is given by
$W=\int F \cdot v d t$, where $F$ is force of friction.
For the rolling disc without slipping down an inclined plane, the velocity of the particle on which the friction force is acting, is zero. Hence, work done is zero, i.e. when the disc rolls without slipping, the friction force is required because for rolling condition, velocity of point of contact is zero.
Therefore, Assertion and Reason are correct and Reason is the correct explanation of Assertion.