Finding the potential energy of a gravitational force?

[CoreanJesus]

Sorry if something is wrong... this is my first ever post
1. Homework Statement
The Question:
In one dimension, the magnitude of the gravitational force of attraction between a particle of mass M1, and one of mass M2 is given by:
F(r)=GM1M2/r^2
Where G is a constant and x is the distance between the particles.

a) What is the potential energy function U(x)? Assume that U(x) -> 0 as x -> infinity.
b) How much work is required to increase the separation of the particles from x=x1 to x=x1+d?

Homework Equations

F(r)=-dU/dx

The Attempt at a Solution

For a), The answer given is -GM1M2/r but I keep getting a positive equation...
∫-dU=∫GM1M2/r^2
-U=-GM1M2/r
U=GM1M2/r
For b) I just need a clue as to how to start...

 

[mfb]

In one dimension, the magnitude of the gravitational force of attraction between a particle of mass M1, and one of mass M2 is given by:
F(r)=GM1M2/r^2

The force is attractive, i. e. towards smaller r. It would be better to add a minus sign here, otherwise the sign convention is confusing.

For b) I just need a clue as to how to start...

What is the potential energy at the described places?

 

[CoreanJesus]

The force is attractive, i. e. towards smaller r. It would be better to add a minus sign here, otherwise the sign convention is confusing.What is the potential energy at the described places?

No potential energy is given...

 

[mfb]

You just calculated it!

 

[CoreanJesus]

Oh :D Thank You!