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.

The friction between the block and the trolley is an internal force (both the action and the reaction are included in the system) and thus their total momentum will not be spoiled.