1. The problem statement, all variables and given/known data A wedge of mass M rests on a horizontal table. A block of mass m rests on the wedge. http://imgur.com/cS02MvM (a) What horizontal acceleration, a, must M have relative to the table to keep the block stationary relative to the wedge, assuming no friction? (b) What horizontal force F must be applied to the system to achieve this result, assuming a frictionless table top? 2. Relevant equations F=(m+M)a 3. The attempt at a solution http://imgur.com/etif1qn All I have so far is this diagram as I don't understand the concept involved. Its not clear to me why the horizontal acceleration of the wedge could keep the block stationary. Also, I'm not sure whether this acceleration would be to the left or to the right. Also I have read through other similar threads but still can not understand the problem.