Fr.: force centrale
A → force that is always directed toward a fixed point and whose → magnitude depends only on the distance from that point. Mathematically, F is a central force if and only if: F = f(r)r1 = f(r)r/r, where r1 = r/r is a unit → vector in the direction of r. If f(r) < 0 the force is said to be → attractive toward the source. If f(r) > 0 the force is said to be → repulsive from the source. In other words, a central force is one whose → potential, V(r), depends only on the → distance from the source. → Gravitational force and → electrostatic force are central, with V(r)∝ 1/r.