Central forces are forces that always act toward or away from a fixed point, with a magnitude that depends only on the distance from that point. A central force F on a particle P can be expressed as F = r f(r), where f(r) is a function of the distance r from the fixed point, and r is the unit vector along the radius. Examples of central forces include gravitational attraction and electrostatic force. Central forces are conservative, have no torque, and cause angular momentum to remain constant.