Pulley Cable System (Conceptual)

[jzwiep]

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.

http://session.masteringphysics.com/problemAsset/1010932/17/MFS_1l_18_001.jpg"

Find the magnitude of the minimum force F that allows the window washer to move upward.

Homework Equations

None that I'm aware of.

The Attempt at a Solution

Based on the F = Tension in the rope

The Tension in the rope holding up the platform is 2T because of the pulley cable system.

This is where I get lost. So the tension in the cable also acts back on the man (as an action-reaction pair)?

Here's a terribly drawn MS paint http://i.imgur.com/lDe6c.png" to better illustrate what I'm asking. Is it right? or even close?

Thanks.

 

[PhanthomJay]

Yes, if the man exerts a force T on the rope, the rope exerts a force T on the man. You are asked to find that force. Note that your picture is missing the rope which acts up on the pulley on the right, supporting that pulley from the ceiling. I am not sure how you arrived at 3T = mg, which is the correct solution given to you?

 

[Chronos]

Try to avoid the illusion a pulley has magical force.

 

[jzwiep]

Yes, if the man exerts a force T on the rope, the rope exerts a force T on the man. You are asked to find that force. Note that your picture is missing the rope which acts up on the pulley on the right, supporting that pulley from the ceiling. I am not sure how you arrived at 3T = mg, which is the correct solution given to you?

The correct answer was: F=Mg/3

Which is just 3T=mg rearranged and with T=F

Am I really that far off?

Try to avoid the illusion a pulley has magical force.

Could you be a little more specific please? I'm not really sure what you're saying. Which pulley did I create an imaginary force at?

 

[PhanthomJay]

The correct answer was: F=Mg/3

Which is just 3T=mg rearranged and with T=F

Am I really that far off?

The answer is correct, I just wasn't sure if it was your answer or just the book's answer.