Assertion : In the absence of external forces, the centre of mass of an isolated system has a constant velocity
Reason : Centre of mass remains unchanged in rotational motion.

Options

A. If both Assertion and Reason are true and Reason is the correct explanation of Assertion
B. If both Assertion and Reason are true but Reason is not correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason are false.

Answer

C

Explanation

The position of centre of mass undergoes a change in translational motion and remains same in rotational motion Applying Newton.s law to the system,`vecF=M (d^2vecr)/(dt^2)` where M is the total mass and , is the position vector of the centre of mass. The external force is zero and the internal forces cancel out in pairs, the net force is zero. Therefore,
`vecF=0implies (d^2vecr)/(dt^2)=0implies (dvecr)/(dt)=`constant
Thus, velocity of centre of mass is constant.