New's Law of Motions Problem!! helppp 1. The problem statement, all variables and given/known data A horizontal force F is applied to a small block of mass m1 to make it slide along the top of a larger block of mass m2 and length l. The coefficient of friction between the blocks is μ. The larger block slides without friction along a horizontal surface. The blocks start from rest with the small block at one end of the larger block as shown http://i37.tinypic.com/2emh3bn.jpg ^will probably have to zoom in on it, click on the image question: Find the acceleration of each block a1 and a2 relative to the horizontal surface. 2. Relevant equations F = ma 3. The attempt at a solution I know the acceleration of a1, that was easy. For a2, I do: fk = m2a2 a2 = um1g/m2 My concern is, in solving for a2 do we use m2 for the system's mass or do we use (m1 + m2) for the system's mass? I still am having trouble understanding that. I guess if you answer this, my concerns will be addressed: Given a situation where there is an object on top of an object and you are finding the acceleration for the bottom object, but the object on top is not moving with the same acceleration as the object on the bottom, that is it is slipping above it, do you account the mass in F = ma as (m1 + m2) because m1 is still on top of m2 no matter what, or do you disregard the upper mass(m1) since it is not moving with m2 as a system.