Question

A child is standing at one end of a long trolley moving with a speed 'v' on a smooth horizontal track. 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. (v + u)
• C. (v - u)
• D. v

Answer 1

The correct option is D v

The speed of the centre of mass of the system will remain unchanged (=v). The speed of the centre of mass of a system can change only if a net external force acts on it. The forces involved (such as the action and reaction and frictional forces), when the child runs on the trolley, are internal to the (trolley + child) system.