Work Step by Step
The force does not necessarily have to be 0. As a matter of fact, the zero of potential energy is whatever we decide to define it as, so the fact that we define potential energy to be 0 at a given point to be 0 does not mean the force is 0. For instance, if I throw a ball up, I could define the zero of potential energy as the peak of the flight path. However, the force would not be 0 at the peak of the flight, for the ball would not come back down to the ground.