Tension Problem: Lifting Yourself

[joshk_bra]

Homework Statement

Lifting yourself. A man stands on a platform as shown in the figure below, and pulls up
on a rope that winds through a frictionless pulley attached to the platform. The other end
of the rope is attached to the ceiling. The mass of the man is m, and the combined mass of
the platform and pulley is M. Can the man lift the platform off the ground if he pulls hard
enough? If so, find the minimum tension required to give the platform a positive (upward)
acceleration, thus lifting it off the floor.


http://i86.photobucket.com/albums/k...yHomework003pdf-AdobeReader.jpg?t=1284942733"

Homework Equations

M = Pulley and Platform total mass

m = Mass of the man

The Attempt at a Solution

Well in class i didn't learn that about tension on a pulley system. But I believe the only forces acting on this is the total weight(m + M), Tension, and Gravity

so i was thinking the equation would loo something like T - Ft = ma?

Im not to sure about my equation, but i was wondering if someone would help me out with this problem

Thanks!

 

[stevenbhester]

I do not think so. If he was to pull on the rope, the force that he puts on the rope would be transferred downwards on the platform he is trying to pull up.

So if he exerts 500 N of force pulling the rope up, it doesn't matter. It's equalled out by force pushing the platform down exerted by the man.

Newton's 3rd law.

 

[joshk_bra]

Well i did more research and from it, i got that the 2 ropes are pulling up on the block and that they are equal( F and T).

so F + T = 2F = mg + Mg or F = (mg + Mg)/2.

Mg + mg is the total weight
Is this correct?