We have now studied the motion of objects, the change in motion resulting from forces acting on objects, and the interchanging of energy from one form to another in the conservation of energy.   We are now about to define a new quantity and get a new conservation principle. As with any conservation principle in physics, it will be of fundamental importance in problem solving.

The linear momentum, p , of an object of mass m and velocity v is simply defined to be

momentum = mass times velocity

p = m v .

The MKS units of momentum are simply as they appear: kg m/s.  There is no fancy name given to this combination of units – we just leave  them in the form kg m/s !

One word of caution about momentum – since velocity is a vector quantity, so must be the momentum. When asked for the momentum of a particle, be sure to give its magnitude and direction (or its components).  When adding momenta (the plural of momentum), be sure to perform the addition by  components!