1. The problem statement, all variables and given/known data Two boxes are connected by a massless string. The first box's mass is 16kg and second box's mass is 21kg. You pull with a total force of 185N on the 16kg box. The coefficient of friction is 0.5. Calculate the tension in the string between the two boxes. 2. Relevant equations Ffr=μFn Fnet = ma 3. The attempt at a solution Thinking of the acceleration of the whole system (including friction) Fnet = Fa - Ffr = ma divide by m to get the acceleration of the whole system. a = (Fa - Ffr)/m m = 16+21 = 37kg Fa = 185N Ffr = μFn = μmg =(0.5)(37)(9.8) a = [185-(0.5)(37)(9.8)]/37 = 0.1m/s2 Using this we should be able to calculate the Fnet of the 21kg box on the end of the string. Fnet=ma = (21)(0.1) = 2.1N Is the Fnet of the 21kg box equal to the tension of the string? I cant seem to get my head around this part. For me it doesnt make sense that even though I'm pulling on the whole system at 185N, only 2.1N is the tension in the string pulling on the 21kg box. Should I have taken the force of friction into account in calculating the Fnet of the 21kg box? Should I be adding or subtracting 185N from something?