# Potential energy problem

1. Jan 15, 2008

### Twilit_Truth

[SOLVED] Potential energy problem

A man that has a mass of 80.0 kg, wearing a backpack with a mass of 20.0 kg, climes 8848 m to the top of Mount Everest. What is his potential energy?

The only one I know of is PE=mgh

I added the masses, then plugged it into the equation, but I didn't get the answer.

Edit:It was already in meters. Sorry.

Last edited: Jan 15, 2008
2. Jan 15, 2008

### Kurdt

Staff Emeritus
Did you convert the height in feet to meters?

3. Jan 15, 2008

### Twilit_Truth

Sorry, it was in meters already. I apparently can't type well today.

Last edited: Jan 15, 2008
4. Jan 15, 2008

### Kurdt

Staff Emeritus
The problem is that equation only holds for short distances above the Earths surface since g depends on the height above the Earth. Have you seen any other formula for gravitational potential energy?

5. Jan 15, 2008

### Tom Mattson

Staff Emeritus
Kurdt, 8848 m is a short distance above the surface of the Earth, so that is definitely not the problem.

Twilit_Truth needs to show some work if we are to figure out what the real problem is. I'm guessing he's either making a calculational error, or doing something silly like using 32 for g, when everything is in mks units. Or possibly it's more subtle. Perhaps he is using a different datum than the answer key is using. IOW, perhaps he is setting V=0 at the surface of the Earth, while the author of the book sets V=0 at the center of the Earth or at infinity.

Twilit_Truth: Let's see how you're doing this.

Last edited: Jan 15, 2008
6. Jan 15, 2008

### Kurdt

Staff Emeritus
haha sorry. Long time since I've done one of these. Guess I was jumping too far ahead on tracking the problem.

7. Jan 15, 2008

### Twilit_Truth

Ok, I'll try to show you what I did. Just so you know, this is a multiple choice question.

First off, I combined the masses of the climber and the pack, resulting in a total mass of 100. kg. Then, I multiplied it by 9.81, which is the only value that I know of for g when on the Earth. This came to 981 N. Finally, I multiplied this by the height, which is 8848 m, and got 8679888 J.

The problem is, the answers are:

A) 6.94E6 J
B) 4.16E6 J
C) 2.47E6 J
D) 1.00E6 J

I don't know where I went wrong. I was absent while they taught this, so I probably messed up somewhere. Thank you both for your time.

8. Jan 15, 2008

### hage567

Why are there 4 answers? Is this question supposed to be part A,B,C or D?

9. Jan 15, 2008

### Twilit_Truth

It's a multiple choice question. Only one answer is right out of those four.

10. Jan 15, 2008

### hage567

Oh I get it.

Anyway, I believe this is somewhat of a trick question. The question asks what is *HIS* potential energy.

11. Jan 15, 2008

### Twilit_Truth

Good eye. Thank you for the help.

12. Jan 15, 2008

### hage567

You're welcome.

13. Jan 15, 2008

### Masest

I did His force, and it came out to be 6943910.4 i figure that is one of your answers