1. The problem statement, all variables and given/known data Box A Mass=3.2kg Force of Friction=2 Newtons Box B Mass=2.1kg Force of Friction=1 Newton Force applied 10.5 Newtons [horizontally]. Q. Find the magnitude of the force of contact between the two boxes. 2. Relevant equations F=ma 3. The attempt at a solution I need to find acceleration first since the Force of Friction of both boxes is not greater than the Force Applied. Fnet=(mA+mB)a (mA+mB)a=Fa-(FfA+FfB) (3.2kg+2.1kg)a=10.5N-(2N+1N) (5.3kg)a=10.5N-(3N) a=10.5N-(3N)/5.3kg a=7.5N/5.3kg a=1.415m/s/s Now, I tired many, many methods of solving for this from here on, including the example from this page:https://www.physicsforums.com/showthread.php?t=202002 However, the result seems still to be wrong. I got 7.5N as the internal force both boxes act upon eachother. Any help is appreciated although not dire.