Guy jumping off a building HELP

[SUXinPHY]

Homework Statement

a 75kg thief wants to escape from a 3rd story jail window.unfortunately, a makeshift rope made of sheets tied together can support a mass of only 58kg. how might he use this "rope" to escape ? give a quantitative answer.

Homework Equations

$$\Sigma F$$ = T - mg

(there might be more, but for me that all i can think of)

The Attempt at a Solution

sry i could not figure it out at all...

 

[montoyas7940]

If one rope was not strong enough what could you do?

 

[SUXinPHY]

erm...not sure

 

[montoyas7940]

Well, if the force on one rope is too great maybe there is a way to divide the force among more than one rope.

EDIT: You are not given a length for the rope. So don't assume it is only three stories long.

 

[SUXinPHY]

meaning?

 

[montoyas7940]

If you look closely at a rope you will see that it is made up of multiple strands of much smaller fibers. You can use this same idea to solve your problem.

 

[SUXinPHY]

as having more then 1 tension..

 

[montoyas7940]

Sure, maybe two "half" tensions. They still add to the total needed but each would be less than the capacity of your rope.

 

[SUXinPHY]

erm so r u saying tat i should use 1/2T + 1/2T - mg= ma

 

[montoyas7940]

No, in this case ma = mg. But don't let that confuse you. Do you understand that all you need to do is consider that half the weight can be carried by each of two ropes to overcome the limitation of one single rope?

 

[SUXinPHY]

tnx a lot i got it now...