# A window washer: tension problem

1. Oct 10, 2008

### DakE_FeatH

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

Fnet=ma
FG=mg

3. The attempt at a solution[/b

I said the forces that are pulling the window washer up consist of a force equal to F and a force equal to the tension in the string attached to the platform.

Then, I took into calculation all the forces acting on the washer, which are the platform, the earth and the cable.

I know that T is upward and F down, and T=|-F|

I need F

I am thinking Fnet is FN + T - FG - F

And I am really stuck :(

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2. Oct 10, 2008

### Redbelly98

Staff Emeritus
I think you can treat the person, platform, and bottom pulley as one object. Then you don't need to worry about the extra variable FN, the normal force of the platform on the person.

So to get Fnet, what are all the forces acting on the "object".

3. Oct 10, 2008

### DakE_FeatH

All the forces acting on the object are T, FG and F

so FNET=T - FG - F ?

or is it T=FG+F
F = T - FG ?

4. Oct 10, 2008

### Redbelly98

Staff Emeritus
F is upward. He pulls down on the rope, so it is pulling up on him (Newton's 3rd Law).

But we're not quite there yet. In how many different places does the rope pull upward on the object?