# Gravitational Potential Energy Question

## Homework Statement

[/B]
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).

## Homework Equations

U=-G(m1*m2)/r[/B]

## 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!

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TSny
Homework Helper
Gold Member
Did you get the right sign for the answer?

Did you get the right sign for the answer?
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..

SteamKing
Staff Emeritus
Homework Helper
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..
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!

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?

SteamKing
Staff Emeritus
Homework Helper
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?
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.

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.
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

TSny
Homework Helper
Gold Member
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
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.

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.
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.

TSny
Homework Helper
Gold Member
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.)

OmCheeto
Gold Member
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
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.

Nailed it!