## Quick Reference

A force acting on a particle, directed towards or away from a fixed point. The fixed point *O* may be called the centre of the field of force. A central force **F** on a particle *P* is given by **F**=*f*(*r*)**r**, where **r** is the position vector of *P* and *r*=|**r**|.

Examples are the gravitational force **F**=−{(*GMm*/*r*^{3})**r**, and the force **F**=−(*k*(*r*−l)/*r*)**r** due to an elastic string (see Hooke's law).

*Subjects:*
Mathematics.