The deformation of the bar, per unit length in the direction of the force (i.e. δl/l) is known as primary strain or linear strain.

When a change in length takes place, the strain is known as linear strain.

Young’s Modulus is defined as ratio of Normal Stress to Longitudinal Strain. Young’s Modulus is usually denoted with letter E. Young’s Modulus is also called as Modulus of Elasticity.

Modulus of rigidity may be defined as the ratio of shear stress to shear strain.

The deformation of a bar under its own weight = Wl^2 / 2E.
whereas Deformation under direct load = Wl^2 / E.
therefore The deformation of a bar under its own weight is half (1/2) the deformation if the same body is subjected to a direct load equal to the weight of the body.