A child sits stationary at one end of a long trolley moving uniformly with a speed V on a smooth horizontal floor. If the child gets up and runs about on the trolley in any manner, what is the speed of the CM of the (trolley + child) system?

Question

Philoid · Accepted Answer

Unlike External forces, internal forces produce no effect on the motion of bodies which they act. Since, child is running on the trolley, the force generate by him is internal to the (trolley + child) system.

So, the speed of the CM of the (trolley + child) has no effect even though the child is running.