1. The problem statement, all variables and given/known data Consider a box sitting on cart. When the cart is pulled with a force, P, the box A moves with the cart without slipping. In all questions, assume there is a negligible amount of drag. There is, however, kinetic friction between the cart and ground. What are the two sets of action/reaction pairs between the cart and Box A? 2. Relevant equations I don't think there is any equations that really relate to this. This is just testing my knowledge of action/reaction pairs. 3. The attempt at a solution So I thought I definitely had one, weight and normal force. The weight of the box acting on the cart and normal force of the cart pushing up on the box. But now I'm questioning if that's even an action/reaction pair because weight is between the Earth and the box right? And then for the 2nd one, I'm thinking static friction, because the box isn't slipping. But then what is the friction's partner? Is it the air pushing on it from the movement? I know this problem is very simple, it's just stumping me to no end. I don't know if I'm making it too complicated in my head or what. Can someone help clear it up for me? Thanks!