1. The problem statement, all variables and given/known data A small mass (1 kg) sits next to a larger mass (3 kg) on a table. A force of 5 newtons pushes from left to right on the system while a force of 3 newtons pushes from right to left on the system. Am I justified to conclude that the net force on the larger block has magnitude 2 newtons? 2. Relevant equations Why can't I get the same answer when I solve separately the force of block two onto block one (the larger onto the smaller)? I get the answer when I solve for F_12 and then state that F_21 is - F_12 by Newton's Third Law. But, why can't I do the reverse, F_21 solved first? Why do I get two separate answers? 3. The attempt at a solution I correctly answer that the answer is no. In fact, the force on block two (the larger block) is actually 1.5 newtons. First, I treat the whole as a single system to get the acceleration, which is a = (F_right - F_left) / (m1 + m2) = 0.5 m/s^2 I then solve for the force of block one onto block two to get F_12 = m2*a = [ m2 / (m1 + m2) ] * (F_right - F_left) = 1.5 newtons (this is the answer) To get the force of block 2 onto block one, I simply chant that because they are an interaction pair, F_12 = - F_21. [Onion]. BUT, if I start the problem solving for the force of block two onto block one, F21, I cannot get the answer. Actually, I get that the F_21 = 0.5 newtons. This does not make any sense. F_21 = m1*a = [ m1 / (m1 + m2) ] * (F_right - F_left) = 0.5 newtons ??? (why isn't it 1.5 N?) My point is that I only get the right answer when I do it one way. When I try different ways to reinforce that I understand, being mathematically poetic, I quickly hit the skids. Thank you and looking forward to see where I went wrong. Sidenote Is it correct for me to write that the force on block one is F_1 = F_right - F_21 = m1*a ? and that that force on block two is F_2 = F_12 - F_left = m2*a ?