A force acting on a particle, directed towards or away from a fixed point. O, the centre. A conservative central force F on a particle P is given by $F=f\left(r\right)r$, where r is the position vector of P and r = |r|.

Examples are the gravitational force F = −(GMm/r³)r, and the force F = −(k(r−l)/r)r due to an spring of unit natural length (see Hooke’s law).