1. The problem statement, all variables and given/known data Two discs of different radii and masses are kept on a smooth horizontal table and both are free to rotate about thier fixed central vertical axis. One of them is given some angular velocity while other is stationary. The rims are brought in contact. There is friction between the rims. Can you conserve angular momentum of system? If the stationary disc is kept above the other disc such that thier centres coincide, will the angular momentum be conserved of the system be conserved? There is friction on the surface of discs. 2. Relevant equations None 3. The attempt at a solution The angular velocity of one disc decreases and the other increases in both cases. Also, Case 1: Friction stops when the rims have have equal velocities and till that moment, friction has equal magnitude on both discs. Case 2: Friction stops when both discs spin with same angular velocity For case 1, torque acting on both discs are different. Net torque is not zero. So angular momentum of system can't be conserved. For case 2, angular momentum is conserved since torque due to friction is same on both discs. Is this correct?