dW

The body is acted on by both external body forces ( ) and the surface forces ( ). The rate at which work is done by the body forces can be represented as,

(3.3)

where the integration is taken through the volume of the unstrained state of the body.

Similarly, the work done by the surface forces is expressed as,

(3.4)

where the integration is taken over the surface of the body in the unstrained state.

Putting the expressions for from equation 2.7 we have,

(3.5)