Not intuitive--friction causes motion? Hi. I'm having difficulty conceptualizing this situation: Consider two objects, mass 1 and mass 2. Mass 2 is on mass 1 and mass 1 is being pulled by a rope. There is a frictional force between mass 1 and mass 2, but none between mass 2 and the surface. Now my confusions/questions are as follows: How is the acceleration of mass 2 caused by the frictional force between the two objects? It is known that friction retards motion, so how does this force supposedly help object 2 accelerate? It would be appreciated if you can help me visualize/provide intuition for this situation (how friction causes acceleration, that is).