# Homework Help: Gravitational Potential Energy Question

1. Mar 20, 2015

### Westin

1. The problem statement, all variables and given/known data

Calculate the gravitational potential energy of the interacting pair of the Earth and a 23kg block sitting on the surface of the Earth. You would need to supply the absolute value of this result to move the block to a location very far from the Earth

(actually, you would need to use even more energy than this due to the gravitational potential energy associated with the Sun-block interacting pair).

2. Relevant equations

U=-G(m1*m2)/r

3. The attempt at a solution

G=6.67*10^(-11)
(mass of the earth)m2=5.97*10^24 kg
m1= 23 kg (as the question)

U = (6.67E-11 * 23 * 5.97E24)/6.378E6 = 1,435,963,782

This was incorrect, any help would be appreciated, thanks!

2. Mar 20, 2015

### TSny

Did you get the right sign for the answer?

3. Mar 20, 2015

### Westin

My answer came out positive, I believe it should be negative cause the question states to take the absolute value of the answer. not sure though..

4. Mar 20, 2015

### SteamKing

Staff Emeritus
The formula for U has a negative sign in it. Didn't see a corresponding negative sign when you plugged numbers into that equation, though.

And don't forget the units!

5. Mar 20, 2015

### Westin

So to get U, i would multiply both sides by a negative, which i would get -1,435,963,782, and then I would take the absolute value of it? so Then it would be 1,435,963,782?

6. Mar 20, 2015

### SteamKing

Staff Emeritus
Well, your formula from the OP was

U = -G(m1*m2)/r

you apparently didn't include the negative sign in front of G in your calculation. You still need to figure out what the units of U are.

7. Mar 20, 2015

### Westin

Units of U are Joules, my answer was 1,435,963,782J but that was wrong, I'm not sure where I am going wrong

8. Mar 20, 2015

### TSny

When doing the calculation, did you take into account the negative sign in the formula for U? Note that the question is asking for U. It is not asking for the the absolute value of U.

9. Mar 20, 2015

### Westin

Yes so then I get -1,435,963,782J , I would leave it as negative then in my answer since its not asking for absolute value of U. I only have last try, could my work be double checked for errors? Thank you for the explanation.

10. Mar 20, 2015

### TSny

Using the numbers you stated in the problem, I get the same answer.

I don't know whether or not you are supposed to think about significant figures here. But, in general, you would want to round your answer to an appropriate number of significant figures. I guess you are putting your answers into some program and maybe you don't need to worry about that.

Anyway, I think your answer is now correct. So, I would say go for it. (You are too far away to punch me in the nose if I am wrong.)

11. Mar 20, 2015

### OmCheeto

That's the answer I get. Are you leaving out something from the problem statement?

"Calculate the gravitational potential energy of the interacting pair of the Earth and a 23kg block sitting on the surface of the Earth."

I've never seen a question ask for the potential energy of an object sitting on the surface of the earth.

12. Mar 20, 2015

Nailed it!