1. The problem statement, all variables and given/known data I got this problem from K&K. A pedagogical machine is illustrated in the sketch above. Refer link : http://www.slideshare.net/brigittperalta/sol-maquina-pedagogica-1546585 . What force F must be applied on M1 to keep M3 from moving up or down. All surfaces frictionless. 2. Relevant equations 3. The attempt at a solution If I go and sit on the M1 block, I can mark a fictitious force on M2 to the left with magnitude (M2.F)/M1. On M2 there is also tension from the string (T) to the right. If M3 is to remain in equilibrium, M3.g = T and T = (M2.F)/M1 to prevent block 2 from moving which will cause block 3 to move. Therefore, I get my answer to be F = (M3.g.M1)/M2. I am making a mistake here but cannot see where. Can someone please tell me where?