A window washer of inertia M is sitting on a platform suspended by a system of cables and pulleys as shown (Figure 1) . He is pulling on the cable with a force of magnitude F. The cables and pulleys are ideal (massless and frictionless), and the platform has negligible inertia.
Assuming this is your figure:
if the rope tension is F, then the total force pulling down on the top pulley is 2F; therefore the total forces pulling down on the overhead support are
F + 2F = 3F.
These must sum to the weight of the window washer (and his accoutrements):
M*g = 3F
F = M*g / 3
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