1. The problem statement, all variables and given/known data The amplitude of the oscillation gradually increases till block B starts to slip. At what A does this happen?(there is no friction between the large block and the surface) 2. Relevant equations Force equations etc... (F=ma) and I reckon it has something to do with the force being combined with that a up there, but I have no clue where to start so that the mass of B cancels. 3. The attempt at a solution No dice :( I hope I've done everything right, I couldn't find an equation editor, so I just used office suit's one and upped the pics of it.