Using a pulley to raise yourself?

[abcdefj]

Homework Statement

In this figure, the man and the platform together weigh 950N. The pulley can be modeled as frictionless. Determine how hard the man has to pull on the rope to lift himself steadily upward above the ground. (Or is it impossible? If so, explain why.)

http://www.webassign.net/serpop/p4-33.gif

Homework Equations

The Attempt at a Solution

My thoughts:
a) If there's a 950N force initially downward and he pulls upward (say 475N), wouldn't the tension in the string still be equal to 950N? In this case, wouldn't the man be pulling upward 475N plus an extra 475N? = impossible.

b) The tension in each string equals the total tension in the system (is this true by the way?), so each string would have a tension of 950/2 = 475N, which is how much the man would have to pull upward. = possible.

 

[Doc Al]

What must the tension be in the rope to support the platform plus man?

What is the relationship between the force the man exerts on the rope and the rope tension?

 

[abcdefj]

Since the man and the platform together weigh 950N (downward force), the tension in the rope must be 950N (upward force).

I'm not quite sure how to go abouts your second question. Isn't the force the man exerts on the rope and the rope tension supposed to be equal?

 

[Doc Al]

Since the man and the platform together weigh 950N (downward force), the tension in the rope must be 950N (upward force).

Good.

I'm not quite sure how to go abouts your second question. Isn't the force the man exerts on the rope and the rope tension supposed to be equal?

Exactly. So how hard does he have to pull?

 

[abcdefj]

He has to pull 950N. Wow, I was just going in circles there! Thanks :)