OK I was helping a friend with his physics HW but I can't remember for the life of me how to do this. A block of mass 4.0 kg is put on top of a block of mass M = 5.0 kg. To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 17 N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table Find the magnitude of the maximum horizontal force F that can be applied to the lower block so that the blocks will move together. Any hints?