1. The problem statement, all variables and given/known data A pedagogical machine is illustrated in the sketch, yatta yatta, what force F must be applied to M_1 to keep M_3 from rising or falling? No friction. Here's the http://www.slideshare.net/brigittperalta/sol-maquina-pedagogica-1546585" [Broken]. 3. The attempt at a solution If we examine the problem from M_1's frame, M_2 seems to be experiencing a force to the left of magnitude $F/(M_1+M_3)\cdot M_2$. The fraction is the acceleration F imparts on M_1 and M_3 (as they are horizontally constrained to be together) and by multiplying it by M_2 we find the (inertial) force on M_2. This should be equal to the force of gravity on M_3 so M_3 g=F/(M_1+M_3)\cdot M_2. Solving for F, we get F=2Mg where M=M_1=M_2=M_3. However, K&K says I should be getting 3Mg. Can anyone explain why F acts on all three masses? Thanks.