It is a spinless particle of mass m moving in three-dimensional space, subject to a central force whose absolute value is proportional to the distance of the particle from the centre of force. F = − − k r It is said to be isotropic since the potential V ( r ) acting on it is rotationally invariant, i.e., V ( r ) = 1 2 m ω ω 2 r 2 where ω ω is the angular frequency given by k / m .