Solve Energy Word Problem: Find Height Swinging Rope w/ 3.5 m/s Vel

[Thenewbie]

Homework Statement

A student is running at a speed of 3.5 m/s and grabs a long rope that is hanging vertically from the ceiling. How high can the student swing?

Homework Equations

For what we're learning right now, we should be using:
PEi + KEi = PEf + KEf

The Attempt at a Solution

I don't even know where to start. What possible formula can help me find this answer with only the initial velocity?

PS. The answer is provided, but I am trying to figure out just HOW to do this. Thanks to somebody who can explain or guide in the right path...

 

[cam875]

hmm that's strange when we did the law of conservation of mechanical energy there was never a question with only one variable given. I am pretty sure that question is impossible unless they expect you somewhere to assume a variable. Check that its not apart of a bigger question or a series of questions that references the other variables that you require.

 

[whybother]

I'd consider using conservation of energy.

At the point he grabs the rope, he has all kinetic energy and no potential energy. By the time the rope reaches its maximum height, it has no kinetic energy and all potential energy.

It's at least a mental starting point for you to try, as long as you know the expressions for kinetic and potential energy.

 

[Thenewbie]

I'd consider using conservation of energy.

At the point he grabs the rope, he has all kinetic energy and no potential energy. By the time the rope reaches its maximum height, it has no kinetic energy and all potential energy.

It's at least a mental starting point for you to try, as long as you know the expressions for kinetic and potential energy.

AHH! Thank you thank you! I just got the answer, wow, don't know why I didn't see it before. Our teacher went through this material really fast, so I'm still trying understand it all a little better.

Thanks for all your help!