The figure shows a simple model of a seesaw. These consist of a plank/rod of mass and length allowed to pivot freely about its center (or central axis), as shown in the diagram. A small sphere of mass is attached to the left end of the rod, and a small sphere of mass is attached to the right end. The spheres are small enough that they can be considered point particles. The gravitational force acts downward. The magnitude of the acceleration due to gravity is equal to .