Question

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?

Answer 1

Trolley and the child make the system. Speed of the CM of this system remains same irrespective of the movements of the child inside the trolley in any manner because CM is affected by external forces only and not by the internal forces.