question_answer 3)

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

Answer:

The forces involved in the given problem are the internal forces of the system. No external force acts on the system when the child runs. So, there will be no change in the speed of the centre of mass of the (trolley + child) system.