The Origin of Force
The motion of particles is determined by the forces acting on them. Forces arise because of the variation of potential energy in the region of a particle. Particles always tend to move to where the potential energy is least.
The particle shown at the local maximum above will force a force tending to move it to one of the two local minima, because the potential energy is less there. This is completely general: Forces act in the direction of decreasing potential energy.
This is why objects fall to the ground, like charges repel and unlike charges attract etc.