I have this problem for homework. Find the magnitude of the minimum force F that allows the window washer to move upward. Express your answer in terms of the mass M and the magnitude of the acceleration due to gravity g. The professor didn't go over multiple pulleys though so I am confused. Now, what I thought was that the force required would be the force of the window washer. Since the pulleys and cables are frictionless and have no mass and the platform has no mass, the man would just have to pull down his own mass times gravity. So with only one pulley I thought that the forces required would just be the mass times acceleration due to gravity (Mg). With 2 pulleys in the system, the force required from the man would be half of that of the system with just one pulley (Mg/2). When I put in that for the answer it was wrong and the feedback was: "The upward force on the platform from the lower pulley is 2T, since the pulley feels an upward force of T from each of two cables." Right now i'm stuck at this point.