This equation computes the y component of the strain, `epsilon_y` in the three dimensional case of Hooke's Law.
The reciprocal term of the equation is 1/E where E is the modulus of elasticity (Young's Modulus) .
`nu` is Poisson's ratio = - (lateral strain / longitudinal strain)
`sigma_x`, `sigma_y`,`sigma_z` are the x, y, and z components respectively of the stress