# Platform and pulley system

1. Feb 3, 2015

### Paola Cavazos

A girl stands on a platform and pulley system. The masses of the platform, girl and pulley are M, m and u respectively. The rope is mass-less. Let the girl pull up on the rope so that she accelerates a upwards. What is the tension of the rope? What is the normal force between the person and the platform? WHat is the tension connecting the pulley and the platform?

I think that I should start from T - F = ma, but I really am kind of lost. I hope someone can help me find the accurate solution.

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2. Feb 3, 2015

### Nathanael

You should explain your thinking when asking for help. How did you get this equation? What is it saying?

3. Feb 3, 2015

### Suraj M

I have a small question Nathanael, I'm a bit confused with the problem too. Will the platform even move?? I'm sorry if this is a stupid question!

4. Feb 4, 2015

### Nathanael

It's not a stupid question. I can see how one would think the platform may not move, because the harder you pull up on the rope, the harder your feet push down on the platform. But the upwards force on the platform from pulling is greater than the increased force downward from pushing with your feet.
For the platform to move upwards (or more specifically, for it to accelerate upwards) there has to a net upwards force acting on the system. If the other end of the rope was attached to the platform, and the person was pulling up on it, then clearly it could not move upwards. But since the end of the rope is attached to the ceiling, an upwards external force acts on the platform (and person) allowing upwards acceleration.
P.S.

5. Feb 4, 2015

### Suraj M

I'm sorry but i still don't get how the upward force would be more than the downward. Could you explain?

6. Feb 4, 2015

### Nathanael

I purposely left the specifics of that detail out so as not to ruin it for the OP :) I will explain it to you in a private message.