Strength of materials deals with the internal effects of the external forces acting on a body.
In mechanics we have so far dealt with rigid body. On the other hand the strength of materials will discuss the internal force analysis of deformable bodies. No body is perfectly rigid. Some deformation takes place under the action of external forces.
Fig. 1.1 shows a crowbar which is used to lift the load W by the force P. The value of P = Wa / b . We have found out the force P which is needed to lift the load W. After finding b P, we must make sure that (i) the bar does not break ; (ii) the bar does not bend too much (without lifting the load).