In our comparison of linear and angular quantities
we suggested that there might be a quantity "angular momentum",
(symbol L) equivalent to linear momentum. 



"Translating" from the linear equation gave us 



We might also expect to find a "principle of conservation of angular
momentum". 



Consider the ice skater shown below. 



With arms and leg outstretched, the skater's initial moment of inertia,
is
relatively large. 

Suppose the skater starts rotating with a low initial angular
velocity ω_{initial}. 

The skater then gradually decreases her moment of inertia
by bringing arms and leg nearer to the axis of rotation. 

Her angular velocity is observed to
increase. 

This is easily explained if we consider that the person’s
angular momentum does not change. 

So, we will suggest that 



This relation is confirmed by experiment. 



Therefore, we can state the principle of conservation of angular momentum 

The total angular momentum of a system remains constant as long as no
external torque acts on the system. 



Click here for two other examples of
the principle of conservation of angular momentum. 
