Conservation of angular momentum can be applied when there is no external torque on the system which is isolated. However, my physics teacher gives me a sum that:
Two discs are pressed against each other until they stop slipping on each other. Find angular speed of the bigger disc when they stop slipping. Disc 1 (mass $m$, radius $r$) and disc 2 (mass $m/2$, radius $2r$). Disc 1 had angular speed $\omega$ initially, whereas disc 2 was at rest initially.
I tried to apply the law of conservation of angular momentum in this case since there is no external torque, but my teacher says that angular momentum is not conserved since there is an external torque acting on it. I fail to understand what is the external torque in this case. I don't need the answer to this question, but the reason why the law of conservation of angular momentum cannot be applied, and what is the external torque that acts on this body?