Angular Momentum: Why Not Conserved w/ Friction?

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When two circular discs of different radii and angular velocities come into contact, friction causes them to eventually move together without slipping. Although angular momentum is conserved in the absence of external torques, the presence of friction leads to energy loss, converting some angular energy into heat. This means that while the total angular momentum of the system remains constant, the mechanical energy is not conserved due to the internal frictional forces. The discussion highlights the distinction between the conservation of angular momentum and the loss of mechanical energy in systems experiencing friction. Understanding this difference is crucial in analyzing rotational dynamics.
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if two circular discs of different radius and angular velocity are brought togather , they experience friction and finally there is no slipping between them , why is the total angular momentum not conserved in this case ? there is no eternal torque , friction is an internal force of the system !
please help!
 
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Angular momentum will be preserved, but not angular energy, some of which is converted into heat by the friction.
 

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For simple comparison, I think the same thought process can be followed as a block slides down a hill, - for block down hill, simple starting PE of mgh to final max KE 0.5mv^2 - comparing PE1 to max KE2 would result in finding the work friction did through the process. efficiency is just 100*KE2/PE1. If a mousetrap car travels along a flat surface, a starting PE of 0.5 k th^2 can be measured and maximum velocity of the car can also be measured. If energy efficiency is defined by...
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