Conservative forces and potential energy

[CmbkG]

Homework Statement

Show that F=-mgk is a conservative force with potential energy V=mgz

The Attempt at a Solution

I don't really know where to ven start with this one. I know it may be something to do with Newton's 2nd law F=ma, but i really don't know what to do

 

[Pere Callahan]

What's the definition of a conversative force and a potential? What is their relation?