# Expression for the gravitational potential Vgrav

1. Feb 26, 2009

### c_m

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

In a system where other gravitational influences can be discounted, two particles of equall mass m, are fixed at positions x= 0 and x= x0 on the x-axis.

1) derive an expression for the gravitational potential Vgrav at a general position x on the x-axis.?

2. Relevant equations

The only equations i can seem to find are:

A) for a test mass m at a distance r from another mass M
Vgrav(r) = (1/m)Egrav = (1/m)(-GmM/r) = -GM/r

B) V(r) = Q/ 4pie e0 r
Where e0 is the permittivity of free space

3. The attempt at a solution

Well to derive an expression usually means i have to combine and rearrange two seperate expressions, but i do not know where to begin, maybe i dont need either of these expressions? could somebody please help me to understand what is goin on here? and prehaps where to start looking?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 27, 2009

### Kurdt

Staff Emeritus
Equation A you have there is the gravitational potential. Equation B is the electrostatic potential so you won't need that. So you have two masses along the x-axis and you want to know the potential along the axis dues to those two masses. How do you think you should proceed from here?

3. Mar 2, 2009

### c_m

Thankyou for relpying

I have actually sent my work now but i did not really do this one, so i would still like to go through it to see what i should have done.

So i do need equation A then? but not B. do i need to find another expression now then for the potential? I just didnt know where to begin.

4. Mar 2, 2009

### Kurdt

Staff Emeritus
When there is more than one mass involved you can sum the potentials.

5. Mar 2, 2009

### c_m

you mean like, V = G m1 m2 / r? rather than V = Gm/r?

6. Mar 2, 2009

### Kurdt

Staff Emeritus
No. Work out the potential using equation A due to one mass, then the other and then add them together to find the potential due to both.