The phenomenon of precession is important and is also
easily accounted for if we understand angular momentum. Here is an
example. A spinning wheel with the rotation axis in the horizontal
plane is suspended from
a string attached to the end of the axis of rotation. The point of attachment is located a distance, d from the center of the wheel.
My point of reference will be the center of the wheel and to be
specific about the vector nature of the quantities involved
I choose a coordinate system with 
vertical and
parallel to 
. The angular
momentum is
The torque about our reference point is
The situation is similar
to the case of uniform circular motion where the net force
acts perpendicular to the momentum of the particle.
Because the change in is oriented along 
we indeed
see that in analogy with the case of circular motion and the centripetal force we have a change in the direction of , not
its magnitude. The change in a time unit dt is :
At time t+dt the angular momentum vector is therefore
The angle between 
and 
is
The angular velocity associated with the precession is obtained by
dividing this equation by dt:
Note that this results holds only temporarily when
ie
when the angular momentum is dominated by the angular
momentum of the rotation of the wheel rather than the angular
momentum associated with the precession around the vertical axis.
In fact the precessing state is not stationary that is the motion continuously is changing and will eventually end up with the wheel
rotating about a vertical axis. This progression is a more complicated phenomenon which is fully described by formulae
which have appeared in this section but which is too complicated for
us to cover in detail here.