Help on finding an expression for the radial force

[Prip_792]

Homework Statement

The potential energy of a system of two particles is inversely proportional to their separation, d. Find an expression for the radial force that each particle exerts on the other.

Homework Equations

The Attempt at a Solution

I'm really stuck on this one guys (and girls). PE=1/d, or at least I'm pretty sure. But how does that relate to the radial force?

 

[Doc Al]

How does force relate to potential? See: http://hyperphysics.phy-astr.gsu.edu/hbase/pegrav.html#pef"