d[Tau]
------ = Q1q`1 + Q2q`2 + ..., (15)
dt
and therefore in the case of a conservative system free from
extraneous force,
d
--([Tau] + V) = 0 or [Tau] + V = const., (16)
dt
which is the equation of energy. For examples of the application of
the formula (13) see MECHANICS, Sec. 22.