here is the scenario: there are 2 blocks A & B, both have the same mass 'm'. block A is kept on an endless surface. there is no friction between the block A and the surface. now the other block, B, is kept on top of the block A. the coeff of friction between A & B is say 'u'. the limiting friction between A & B is therefore = umg = 'f'. now say i apply a force 'F', equal to 'f', on the block A, i.e F = f. What will be the FBD's for A and B. I did the FBD for B & A and i am getting f = 0 = F , which is not possible since f = umg. So I think i have made some mistake. And I am not saying that A and B are necessarily stationary initially, they could be moving with a uniform velocity. Is this suggesting that since there is no relative motion between A & B therefore there is no friction existing. If this is the case then isnt this contradicting the fact that B moves with A (with a relative velocity = 0) due to friction when a force is applied to A and that if friction was not present A would just slide past underneath B and B would just fall to the ground. kindly help me out with this! Thanks.