[Sigma](X[delta]x + Y[delta]y + Z[delta]z) = Q1[delta]q1 + Q2[delta]q2 + ..., (7)
/ dPx dPy dPz \
where Q_r = [Sigma]( X ----- + Y ----- + Z ----- ) (8)
\ dPq_r dPq_r dPq_r/
The quantities Q1, Q2, ... are called the _generalized components of
force_ acting on the system.