There's two force problems I couldn't get on my study guide. 1. Two blocks (A and B) are in contact on a horizontal frictionless surface. A 36-N constant force is applied to A as shown. The magnitude of the force of A on B is: There is a picture. Block B is on the right of Block A. 36N is applied to block A. Block A = 4.0kg and block B = 20kg. A. 1.5 N B. 6.0 N C. 29 N D. 30 N E. 36 N What I did is: F = (mA + mB)a a = 36 / (4.0 + 20) a = 3/2 m/s2 FAB = 4.0(3/2) = 6.0 N The correct answer is D. 30N. I can see where it comes from. I thought "the magnitude of the force of A on B" means I should multiply mA and a to get the force, but to get 30 N they multiplied mB and a. Why? 2. Two blocks with masses m and M are pushed along a horizontal frictionless surface by a horizontal applied force F as shown. The magnitude of the force of either of these blocks on the other is: In this picture. M and m are in contact. m is on the right of M. Force F is applied to M. A. mF/(m + M) B. mF/M C. mF/(M - m) D. MF/(m + M) E. MF/m The correct answer is A. I don't even know how to start this problem. I tried to solve for a and substitute but that failed. Any ideas? These came from a study guide I was given in class. I thought I knew this topic well but these problems really scare me. Can someone please explain these to me? I have a test coming up pretty soon. I read this section in the book, but sometimes I can't apply what I read to every problem.