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 centre of mass of the (trolley + child) system?
Solution
In the absence of external forces, the centre of mass moves with constant velocity. That is, the velocity of the centre of mass of trolley and child remains unchanged, i.e., v.
When the child is moving on the trolley, the force exerted by the child on the trolley is the internal force.
When the child is moving on the trolley, the force exerted by the child on the trolley is the internal force.
