Surfaces forces - like tires skidding on a highway or the forces acting across an internal surface of any orientation that divides a body. Second, we need to be accurate about the units we use. It's also of use in structural geology to think of the force intensity or force per unit area. These symmetrical tractions, in total, are the surfaces stresses. So surface stresses are like tractions, but they are symmetrical about the surface of interest.
Surface stress can therefore also have normal stress and shear stress components.
This relation shows that both the traction and surface stress are dependent on the surface area over which they act. This is the characteristic that distinguishes stress from force . As a result of this difference, a transformation equation is needed.