1. The problem statement, all variables and given/known data "Three blocks (m1, m2 and m3) are in contact with each other on a frictionless, horizontal surface. A horizontal force is applied to m1. http://i4.photobucket.com/albums/y111/kathy_felldown/sb-pic0556.png What is the net force on block 1 and block 2? What is the magnitude of the contact force between blocks 1 and 2?" 2. Relevant equations Fnet = ma 3. The attempt at a solution I found acceleration by calculating the Fnet = m(total)*a a = 17.9 / (1.61 + 3.39 + 4.16) = 1.95m/s/s Just for curiosity, I tried the Fnet (m1) = (1.61kg)(1.95m/s/s) = 3.14N and it was correct. I have no idea why the applied force, 17.9N is not considered in this... OR the force of m2 on m1 (according to Newton's 3rd law) I also found the Fnet (m2) = (3.39)(1.95) = 6.61N. Why is it that the net force on each mass is simply equal to that object's mass * acceleration? Also, I tried finding the magnitude of the contact force between m1 and m2. I figured that since the two forces are going in opposite directiosns, I should subtract Fnet(m2) - Fnet(m1) = 6.61 - 3.14 = 3.47N... but that is wrong! Also for the contact force between m3 and m2, would the force acting on m2 be the applied force of 17.9N, or the net force that m1 exerts?