1. The problem statement, all variables and given/known data When a block is dropped to a disc that is rotating with a constant angular velocity about its centre, at the end, we know that both of them will rotate with the same new angular velocity which is slower than the previous one. Question: What is the force that makes the block to start rotating? 2. Relevant equations 3. The attempt at a solution Is it friction? Can friction actually starts a motion of an object? I know that it is friction that slows down the disc but what force makes the object to start its rotational motion? I don't know.