An inverse-square central force acting on a single particle is described by the equation F ( r ) = − − k r 2 r ^ ^ ; The corresponding potential energy is given by V ( r ) = − − k / r . The constant parameter k describes the strength of the central force; it is equal to G ⋅ M ⋅ m for gravitational and − k e ⋅ Q ⋅ q for electrostatic forces. The force is attractive if k > 0 and repulsive if k < 0.