In many ways, an introductory physics course is designed to present the fundamental quantities that physicists have learned over the years to keep track of.  The two most important of these quantities that we have seen so far are energy and momentum.  Because these concepts are so fundamental to the way the universe works, it is hard to define them in words.  Instead, we tend to hide behind equations when we need to define energy and momentum.  More importantly, we learned how to keep track of the energy or momentum of a system and that, under certain conditions, energy and momentum can be conserved.

We have reached the point in the course to introduce our next fundamental physical quantity, angular momentum.  As we have seen time and again when discussing the rotational motion of a rigid body, angular momentum as a concept is the rotational analogue of linear momentum.  And just like linear momentum, it is hard to describe angular momentum in words.  We can describe it using equations, but our goal is to learn how to keep track of the angular momentum of objects.  Rigid bodies that rotate will have some angular momentum about their axis of rotation.  But even point masses moving in a straight line can have angular momentum.  Finally, for a system of rotating objects, the angular momentum of the system can be conserved under the right conditions.