Central force
In classical mechanics, a central force on an object is a force that is directed towards or away from a point called center of force. F ( r ) = F ( r ) r ^ {\displaystyle \mathbf {F} (\mathbf {r} )=F(\mathbf {r} ){\hat {\mathbf {r} }}} where F is a force vector, F is a scalar valued force function (whose absolute value gives the magnitude of the force and is positive if the force is outward and negative if the force is inward), r is the position vector, ||r|| is its length, and r ^ = r / ‖ r ‖ {\textstyle {\hat {\mathbf {r} }}=\mathbf {r} /\|\mathbf {r} \|} is the corresponding unit vector.