1. The problem statement, all variables and given/known data I think I am lacking some concept. I can't solve this one: There are three blocks of masses 2kg, 3kg and 7kg lying on a frictionless table with the 7kg block on bottom 3kg in the middle and 2kg on the top. A horizontal force of 10N is applied on the bottom block. The coefficient of friction between the top block and the middle block is 0.2 and that between the middle block and bottom block is0.3. Find the accelerations of all the three blocks. Take g=10m/s^2. Please help. 2. Relevant equations Newton's 2nd and 3rd laws and max static friction=uN where N is normal force and u is coefficient of friction. 3. The attempt at a solution The max value of static friction between the bottom block and the middle block = 0.3*(2+3)*g =0.3*5*10 =15N Since force on the bottom block is only 10N , so the bottom block and the middle one cannot have relative motion, so they must move together. Let their common acceleration be 'a' and the friction between them be F1, then for the bottom block, 10-F1=7a, ................(1) Now, for the middle block the forces, the acceleration of this block must also be 'a' because it is moving together with the bottom one. The forces on the middle block are 'F1'(forward) and F2 ( the frictional force from the top block in the backward direction). So, for the middle block, F1-F2=3a, ................(2) and, for the top block, Let it's acceleration be 'b' . Now, the only forward force on the top block is F2 which is the friction between it and the middle one. So, for the top block, F2= 2b ...................(3) I can't figure out what to do further. I don't know where I am supposed to use the coefficient of friction between the top and the middle block. If I don't know the value of F1, then how am I supposed to know if force on the middle block is greater than or less than the limiting static friction between the top and the middle block (which is 4N). So, how can I know if there will be relative motion between the top and the middle block or if the top one will also move with the same acceleration 'a'?