S = lim F/A 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; a Newton is defined as the force necessary to
accelerate a one kilogram mass by one meter per second, per
second. Twiss and Moores¹ note that the weight (which of course
is a force = mg, where g = acceleration due to gravity) of an
apple is about one newton! A pressure of 1 bar (nominally,
atmospheric