Degrees of Freedom

                         In order to describe completely the motion of a particle in one plane, only two quantities must be known, say its two rectangular components. Similarly, for a particle moving in space, three independent quantities must be known to describe its motion. A molecule in a rigid body can have three motions of vibration along any of three co-ordinates axis in addition to its three motions of translation. It is thus obvious that in order to completely describe the state of motion of a particle, six independent quantities must be known.
                       In general, the total number of independent quantities, which must be known for describing completely the state of motion of a body, are called its degrees of freedom.