Two horizontal discs of different radii are free to rotate about their central vertical axes. One is given some angular velocity, the other is stationary. Their rims are now brought in contact.There is friction between the rims. Then 

a) 
The force of friction between the rims will disappear when the discs rotate with equal angular speeds 
b) 
The force of friction between the rims will disappear when they have equal linear velocities 
c) 
The angular momentum of the system will be conserved 
d) 
The rotational kinetic energy of the system will not be conserved 
Two horizontal discs of different radii are free to rotate about their central vertical axes. One is given some angular velocity, the other is stationary. Their rims are now brought in contact.There is friction between the rims. Then 

a) 
The force of friction between the rims will disappear when the discs rotate with equal angular speeds 
b) 
The force of friction between the rims will disappear when they have equal linear velocities 
c) 
The angular momentum of the system will be conserved 
d) 
The rotational kinetic energy of the system will not be conserved 
(b,d) The force of friction between the two surfaces in contact disappears when there is no relative (linear) motion between them. Angular momentum will not be conserved as the discs will have the final angular velocities in the opposite directions 