My physics core is a little bit weak, and this one question confuses me. 1. The problem statement, all variables and given/known data A 3kg box sits on top of a 7 kg box. The object accelerates at 1 m/s^2. Find the force applied on the bottom block; if there are no forces besides this one. 2. Relevant equations F= ma 3. The attempt at a solution So, my initial attempt was F = m * a. m = 7kg + 3kg = 10 kg. F = 10 N. However, this is incorrect. The answer is supposed to be 3 kg. They only take the bottom block into account and this is extremely confusing for me. Of course, this is from an even larger kinematics question; but this is the only step I do not understand. And it's basically exactly the same as the one I'm asking now. Why do we use 3 kg, and not 10 kg?