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Question: Is the frictional force a force of constraint? What is the virtual work done by frictional forces?
The principle of virtual work and D' Almebert's principle eliminate constraint forces making use of the fact that these forces do no work. What about friction? Is it a force of constraint in this sense? You find discussion of such fine details only in work of masters. See Sommerfeld \(\S\) II.8, p54.
Sommerfeld writes:
... we shall talk about the force of friction, which must be sometimes counted among the forces of reaction, sometimes among applied forces. It is a force of reaction if it occurs as static friction; applied force if it occurs as sliding or kinetic friction. Static friction is automatically eliminated by the principle of virtual work; kinetic friction must be introduced as an applied force. An external indication of this is the occurrence of the experimental constant \(\mu\) in the law of sliding friction.
