# How to convert KE into potential gravitational energy?

1. Dec 5, 2015

### Byron Anderson

Here is the question with two parts

The fastest that a human has run is about 12 m/s.
a) If a pole vaulter could run this fast and convert all of his or her kinetic energy into gravitational potential energy, how high would he or she go? 7pts
b) Using the 1990 pole vault world record of 20 ft. Find the initial speed in m/s needed for the pole vaulter to reach this height. 6pts
(This is Problem 24 of chapter 3.)

Now I know how to get the KE= 1/2md2. Now I am really confused because the question isn't giving me the mass or anything to work with? How do you even go about starting to solve this problem?

On top of that I have no idea how to solve for gravitational potential energy.

Haven't even tried to start the second part b) yet.

I am not looking to get spoon fed an answer I simply need a little guidance for what I am missing. Doing an online class so it's not easy to sit down with the instructor.

2. Dec 5, 2015

### SammyS

Staff Emeritus
Hello Byron Anderson. Welcome to PF !

Use m for the mass. It will cancel out of the equations.

Are you sure you haven't covered gravitational potential energy? Maybe it was simply called potential energy.

3. Dec 5, 2015

### Byron Anderson

If potential energy is the same yes we have. I guess I just didn't get a good hold on it. I will take a look back at potential energy right now and try again.

When you say use m for mass and it will cancel out what does that mean? How can I solve the equation if I still have m as a variable?

4. Dec 5, 2015

### Byron Anderson

Maybe I am asking the wrong question. If PE= work done = weight x distance raised how do I get all the needed variables to solve this?

5. Dec 5, 2015

### SammyS

Staff Emeritus
For a mass of m, the weight (force due to gravity) is mg. Right?

So to lift an object to a height h, requires doing an amount of work = mgh against gravity.

6. Dec 5, 2015

### Byron Anderson

I don't know to make sense out of any of that with just the runner can go 12 m/s? So I know their velocity what about the mass or height or anything else to help me solve it? Seems the question is missing something?

7. Dec 5, 2015

### SammyS

Staff Emeritus
The question is missing nothing.

Have you learned at all about potential energy?

8. Dec 5, 2015

### Byron Anderson

I guess not

I was looking for some direction or help I have these formulas to read in my book seeing them again isn't helping. I am confused on how to plug in this 12 m/s to get an answer?

9. Dec 5, 2015

### SammyS

Staff Emeritus
Earlier, you mentioned something about Work. What do you know about work?

Also, for an object of mass, m, and speed, v, what is the kinetic energy?

10. Dec 5, 2015

### Byron Anderson

work = f/d right? I don't have either of those to use

So I google it and might have found what I was looking for. I didn't see this anywhere in the text book but would height = v squared / 2g? SO 12x12 / 2x9.8? My height is 7.35?

11. Dec 5, 2015

### SammyS

Staff Emeritus
Actually, Work = F⋅d .

Energy can be though of as a property of an object which can be converted into work.

Kinetic Energy, KE = (1/2)m⋅v2 , for mass, m, and speed, v .

It takes an amount of work = m⋅g⋅h, to raise an object a height, h, g being acceleration due to gravity.

∴ (1/2)mv2 = mgh .

The mass m cancels out and you can find height, h, as you did. What are the units for your answer?