
Classical Mechanics50
© 2010 Taylor & Francis Group, LLC
Differentiating Equation 2.43 with respect to time, we nd the second law of motion for the
system as a whole:
e
. (2.44)
That is, the total momentum
of a system of particles changes only as a result of the action of
external forces. Hence, if
e()
, we get the momentum conservation law for a system of particles:
P
(2.45)
Because the velocity of the CM of a closed system of particles is constant in time, the CM frame
is an inertial frame. The description of phenomena in this frame of reference eliminates complica-
tions arising from the motion of the sys ...