What is Shear Modulus (Modulus of Rigidity) and how is it related to Young's Modulus?

Shear Modulus (G) is the ratio of shear stress to shear strain within the elastic limit, measuring a material's resistance to shape change. It is related to Young's Modulus by G = E / [2(1 + ν)], where ν is Poisson's ratio. For steel with E = 200 GPa and ν = 0.3, G is approximately 77 GPa.