Hi, this seems to be an easy enough problem, but for some reason I'm having problems with it. 1. The problem statement, all variables and given/known data A block slides down an inclined plane, and the inclined plane slides horizontally. There is friction at the incline's upper surface, but not at its lower surface. For each body, draw a free body diagram. 2. Relevant equations - none, we only need to draw the free body diagram 3. The attempt at a solution I have already identified the usual suspects: Forces on plane: - the force of gravity, acting downwards - the normal force, acting upwards - Additionally, I know that the plane is sliding along a level floor. Because there is no friction (stated in the problem), there is no force resisting the motion, and, assuming that it slides at a constant velocity, that means that there is no horizontal force acting on it at all (because the horizontal component of the net force must be zero) - the weight of the block also acts on the plane, so we have another force acting downwards here. Here comes the tricky part: I'm not really sure what makes the block move in the same direction as the plane. Is the plane pushing it? What force is that? I understand the other forces on the block (gravity, normal force, friction), I just don't understand what makes it "go along" with the plane. Am I making any sense? Thanks.