Question

A child is standing at one end of a long trolley moving with a speed $v$ on a smooth horizontal floor. If the child starts running towards the other end of the trolley with a speed $u$, the centre of mass of the system (trolley + child) will move with a speed

• A. zero
• B. $\left(v+u\right)$
• C. $\left(v-u\right)$
• D. $v$

Answer 1

Answer: (D)

The speed of the centre of mass of the system remains unchanged (equal to $v$) because no external force acts on the system. The forces involved in running on the trolley are internal to this system.