Find Min Force for Window Washer: Ma = F + 2T_1 - Mg

[clarineterr]

Homework Statement

A window washer of mass M is sitting on a platform suspended by a system of cables and pulleys as shown . 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 mass.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.

Homework Equations

Newton's second Law

The Attempt at a Solution

For the person: Ma = F + T -Mg where T is the tension in the cable holding the platform and F is the force of the rope on the washer.

Then from the lower washer T = 2T_{1}, where T_{1} is the tension in the rope

So Ma = F + 2T_{1} - Mg

The block is about to move so Ma = 0 and I think T_{1}=F so then

0 = 3F - Mg

I am not sure about this though

 

[Doc Al]

Looks good to me.