Differentiation of potential energy

[zcabral]

Homework Statement

The potential energy of a system of two particles separated by a distance r is given by U(r) = (A)/(r^4), where A is a constant. Find the radial force that each particle exerts on the other. (Use A and r as appropriate in your equation.)

Homework Equations

F=-dU/dx
U is potential energy

The Attempt at a Solution

i figured out the antiderivative of U(r) is -(A)/(3*r^(3)) but it isn't the correct answer.
what else do u need to do besides the antiderivative which is equal to force. since it is 2 particles should i divide the U(r) funciton by 2?

 

[tongpu]

you actually want the partial derivative of r

 

[zcabral]

ok but how do u get that? I am rather new at the whole calculus thing. wud it just b A/3r^3?

 

[Feldoh]

Think of U(r) as A*(r^-4) then try to differentiate it...