The number of degrees of freedom of a body is the minimum number of independent coordinates required to describe the position of the body at any instant, relative to a frame of reference. A particle in straight‐line motion or circular motion has one degree of freedom. So too does a rigid body rotating about a fixed axis. A particle moving in a plane, such as a projectile, or a particle moving on a cylindrical or spherical surface has two degrees of freedom. A rigid body in general motion has six degrees of freedom, say, three to specify the position of the centre of gravity and three more to orient the body.