Question

A system of particles is free from any external force. If $\vec{v}$ and $\vec{a}$ be the velocity and acceleration of the centre of mass, then it necessarily follows that:

• A. $\vec{v}=0 ; \vec{a}=0$
• B. $\vec{v} \neq 0 ; \vec{a}=0$
• C. $\vec{v}=0 ; \vec{a} \neq 0$
• D. None of these

Answer 1

Answer: (D)

Absence of external force, only confirms about the fact that $\vec{a}=0$. However, $\vec{v}$ may or may not be zero.