CBSE Class 11-science Answered

Like force and velocity, stress is a vector quantity. The stress vector can be resolved into a component perpendicular to the surface (the normal stress), and a component parallel to the surface (the tangential, or shear stress).

The stress field about any point in the body may be isotropic (also called hydrostatic) or anisotropic (non-hydrostatic). If we imagine a simple, infinitesimal cube, with its sides oriented orthogonally to the three cartesian coordinates, we can define a normal stress, S1, S2, S3 perpendicular to each pair of sides of the cube. These stresses are defined so that S1>S2>S3. Note that stress has units of Pascals = N/m², (where N = newton). The Pascal is the SI unit of pressure.

Pressure

When the stress field is hydrostatic, S₁=S₂=S₃ = P, where P = pressure. Pressure is a scalar (non-directional) quantity; stress is directional. Only when the stress field is isotropic does a thermodynamic pressure exist.