How can we describe "the motion" of, say, a moving car? We have already described its motion in terms of the Kinetic Energy,
K = (1/2) m v2
Another way to describe its motion is in terms of its momentum,
Momentum = mass x velocity
p = m v
Notice tha momentum is a vector; there is a direction to the momentum. That was not true for Kinetic Energy.
Newton's Second Law, which we have written as
F = m a
can also be written in terms of momentum,
F = m a = m (dv/dt) = d(mv)/dt = dp/dt
F = dp/dt
The force acting on an object equals the time rate of change of the momentum of that object.
ToC Conservation of Momentum Return to ToC, Ch9, Linear Momentum (c) Doug Davis, 2001; all rights reserved