Drop an object on a moving trolley
The "Start" button will become the "Drop" button after it is pressed.
The ground on which the trolley moves is frictionless.
Conservation of linear momentum:
When the objects in a system interact, their total momentum remains constant, provided that there is no external force on the system.
In this example, the friction between m and M is an internal force (both the force and its reaction are included in the system).