# Need Help on Gravitation Potential Energy Problem

1. Apr 2, 2008

### brett812718

[SOLVED] Need Help on Gravitation Potential Energy Problem

1. The problem statement, all variables and given/known data
(a) A 5.3 kg particle and a 3.0 kg particle have a gravitational attraction with a magnitude of 2.6 x10^-12 N. What is the gravitational potential energy of the two-particle system?

(b) If you triple the separation between the particles, how much work is done by the gravitational force between the particles?

(c) How much work is done by you?

2. Relevant equations
F=(GmM)/r^2
U=-(GmM)/r

3. The attempt at a solution

First I used the given information to solve for r using newtons law of gravitation. Then I pluged r=1.96E1 into the potential energy equation to get -5.41E-11J but that was wrong.

2. Apr 2, 2008

### Hootenanny

Staff Emeritus
Recheck your arithmetic, you separation value is incorrect.

3. Apr 2, 2008

### brett812718

is R=2.012E1 and U=-5.25E-11 correct

4. Apr 2, 2008

### Hootenanny

Staff Emeritus
Correct indeed

5. Apr 2, 2008

### brett812718

Also would the solution to part b be U=-(GMm)/3r

6. Apr 2, 2008

### Hootenanny

Staff Emeritus
Not quite, the work done is change in gravitational potential energy.

7. Apr 2, 2008

### brett812718

Work=((-GMm)/3r)-((-GMm)/r)

8. Apr 2, 2008

### Hootenanny

Staff Emeritus
Much better

9. Apr 2, 2008

### brett812718

I got 3.5E-11J but that was also incorrect

10. Apr 2, 2008

### Hootenanny

Staff Emeritus
I get something different. Furthermore, be careful with the sign.

11. Apr 2, 2008

### brett812718

I got ((-GMm)/3r)=-1.75E-11J and ((-GMm)/r)=-5.25E-11J
is((-GMm)/3r)=-1.75E-11J correct?

12. Apr 2, 2008

### Hootenanny

Staff Emeritus

13. Apr 2, 2008

### brett812718

work=-3.50E-11J

14. Apr 2, 2008

### brett812718

because work is equal to the negative change in potential energy

15. Apr 2, 2008

### brett812718

and part c is 3.50E-11J

16. Apr 2, 2008

### Hootenanny

Staff Emeritus
Correct.

17. Apr 2, 2008