From Goldstein:
A virtual (infinitesimal) displacement of a system refers to a change in the configuration of the system as the result of any arbitrary infinitesimal change of the coordinates $\delta\mathbf{r}_i$, consistent with the forces and constraints imposed on the system at the given instant $t$.
From my Mechanics instructor (not verbatim):
A virtual displacement is a displacement consistent with the system's constraints.
My instructor's definition of virtual displacement is crystal clear and Goldstein's isn't so I can't determine if they're compatible. What does Goldstein mean by "consistent with the forces"? I find this phrase to be extremely ambiguous. After clearing that up, please discuss the equivalence or lack thereof of these two statements.
