A playground merry-go-round of radius 3 m has a moment of inertia 200 kg.m^2 and is rotating at 10 rev/min about a frictionless vertical axle. Facing the axle, a 40 kg child hops onto the merry-go-round, and manages to sit down on the edge.
The child initially is not on the merry-go-round. I think when the child hops onto the merry-go-round, he will generate an external force acting on the merry-go-round. Why is the net angular momentum of the system remains constant?