Momentum
The momentum of a mass 
is usually defined by
is the velocity of the mass. Note that momentum is a vector
quantity in general, so we should more clearly write
and
are vectors in 3D space. We will
return to vector momentum in §B.4.1 below.
Conservation of Momentum
Like energy, momentum is conserved in physical systems. For example, when two masses collide and recoil from each other, the total momentum after the collision equals that before the collision. Since the momentum is a three-dimensional vector in Euclidean space, momentum conservation provides three simultaneous equations, in general.B.10
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