1. The problem statement, all variables and given/known data A block of mass m rests on top of a block of mass M, which in turn rests on a horizontal surface. The coefficient of static friction between m and M is mu-1 and the coefficient of kinetic friction between m and M is mu-2. The coefficient of static friction between M and the surface is mu-3 and the coefficient of kinetic friction between M and the surface is mu-4. What is the maximum horizontal force you can apply to block M such that blocks m and M move at constant velocity and m does not skid off M. 2. Relevant equations Newtons 2nd and 3rd law.. 3. The attempt at a solution Well, i solved the problem but i am still not convinced my answer is right.. I am assuming the problem i have is with newtons 3rd law (deciding whether the static friection should point right or left). Here is my answer. First, i made a free body diagram from the top block (m). The force down is mg, therefore the upward normal force is the force block M exerts on block m. I put the force of static fricition point left, because without fricition, nothing will keep the block moving to the left. Therefore, the force block M exerts on m (x-direction) is to the right. Therefore, F (M on m in y) = mg UP F (M on m in x) = (mu-1)mg RIGHT Now going to the FBD for the large box, Obviously, F points left, force of kinetic friction points right, Mg points down, and n points up. Now using N3L, F (m on M in y) = mg DOWN F (m on M in x) = (mu-1)mg LEFT Solving this i am left with F(max) = (mu-4)(M + m)g - (mu-1)mg But conceptually, i would think increasing mu-1 should increase the F max..... so i doubt my answe is right.