Are Frictional Forces Always Shear?

Question

Is frictional resistance always shear? Two examples I am thinking of are the cases of friction between solid bodies and "friction" in viscous resistance?

Answer 1

Viscous resistance can be normal to or along the surface. Resistance that is normal to the surface is usually called pressure, and the ones that are parallel to the surface is usually called shear.

Viscous stress tensor : https://en.wikipedia.org/wiki/Viscous_stress_tensor The diagonal elements represents the normal pressure and the off-diagonal elements are the shear forces.

Answer 2

Friction between solids is an adhesive process not a shear process. For a discussion of this see How is frictional force dependent on normal reaction?

However in most cases energy is also dissipated due to elastic deformation of the materials involved. The frictional force at the interface between the two materials causes elastic deformation of the bulk due to the shear stress. Although this is elastic in the sense that no permanent deformation is produced, energy is dissipated as the material flexes. So in this sense shear is involved even in solid friction, though it not a shear flow as it would be in a liquid.

Answer 3

In fluid flow past an object, the force per unit area at each location on the surface can be resolved into a normal component and a shear component. The integral of the normal components over the surface (resolved in the flow direction) is usually designated as the "form drag" and the integral of the shear components over the surface (resolved in the flow direction) is usually designated as "frictional drag." The shear components are strictly viscous, but the normal components are comprised of a combination of viscous stresses and pressure.