1. The problem statement, all variables and given/known data Box B is on top of box A. Box A has a mass of 1 kg, B has 0.3 kg. Coefficient of friction between ground and box A is 0.5, between box A and B : 0.75. With what maximum force can I pull box A without box B sliding back ? m1 = 1 kg m2 = 0.3 kg u1 = 0.5 u2 = 0.75 2. Relevant equations Ffr = umg F = ma 3. The attempt at a solution I got that the force of friction between box A and ground is 6.5 N ( (m1 + m2)u1g ) Between box A and B friction is 2.25 N. ( m2u2g ) So the total force of friction box A receives is 8.75 N. But I can't work out what is the maximum force with which I can pull box A without having box B falling off.