Negative sign when finding forces from derivative of potential?

[21joanna12]

Hi! I'm currently reading a book where they give the Coulomb potential, gravitational potential and harmonic potential as

+Q1Q2/4∏εx

-Gm1m2/x

+(1/2)qx²

I think I get the signs as they are used here, but when I am trying to find the force by taking the derivative of these with respect to x, I don't know whether to to f= +dV/dx or f= -dV/dx for any of them and I can't figure out the significance of the sign- which way is defined as +ve/-ve and why?

Sorry if I posted this is the wrong place or if it is not very clear :/ This is my first post of PF...

Thank you for any replies! :)

 

[MisterX]

F = -dV/dx

This way the force points "down" the slope to a lower potential.

 

[Doc Al]

You might find this short discussion helpful: Negative Signs in Potential

 

[21joanna12]

Thank you! :)