Let   resultant of all the external forces acting o n a body [Fig. 8.1 (a)]

H_G= Angular momentum of centre of gravity (G) of the body

= Acceleration of the mass centre (G) of the body.

From Newton’s Second law of motion we can write

Where                                      m = mass of the body.

Again summation of all the moments  about the mass centre is equal to the time rate of change of the angular momentum

We know that the external forces may be expressed as actihg at the G coupled with  as shown in Fig. 8.1 (b).

Fig. 8.1 (c) shows the kinetic diagram using equations (8.1) and (8.2).