A small block of mass m slides on a long horizontal table when it encounters a wedge of mass 2m and height h The wedge can also slide along the table. The mass slides up the wedge all the way to the top and then slides back down, never getting over the top. All surfaces are frictionless. Consider three moments in time: t1 – when the block is on the table, before sliding up the wedge; t2 – when the block is at the top of the wedge; t3 – when the block is on the table, after sliding down the wedge. The speed of the wedge is: The greatest at t3 (CORRECT) Zero at both t1 and t3 The same but not zero at both t1 and t3 Zero at t2 My answer to part 1 is justified because: The momentum of the wedge-and-block system is conserved. 3. The attempt at a solution first of all I didn't understand how the momentum is conserved? momentum is only conserved when no external forces act on the system( here: block and wedge).But gravity(ext) (mgsin(theta)) is definitely acting on the block. secondly I understand why the speed will be greatest at t3 (because acceleration will be down the wedge and it will increase the velocity) I don't understand why the other option "ZERO AT t2" is wrong.won't it momentarily stop at the top of the wedge. thirdly,the hint to this problem said "We could write an expression for the total momentum of the system at t1, t2, and t3." How would we do that? I really can't wrap my head around the fact that the wedge is moving too. I dealt with a similar problem (no wedge but a slab) which used the concept of centre of mass. Can we "conveniently" use it here. How is the height of the wedge useful?