Book: Variational Principles of Mechanics by Lanczos (page 80)
Statement: "Two systems of forces which produce virtual displacements are dynamically equivalent."
I don't understand the part about forces producing virtual displacements. I think of virtual displacements as infinitesimal variations from a path between two points on the configuration space. But these variations are arbitrary. How are the forces, which are certainly not arbitrary, producing these virtual displacements?