Final answer:
Angular momentum can be conserved or not conserved in a collision depending on the presence of external torque. One example of conservation of angular momentum is when a disk adheres to a stick and rotates about a nail. Angular momentum is not conserved when there is an unbalanced external torque during a collision.
Step-by-step explanation:
Angular momentum is conserved when the net external torque is zero. One example of a collision in which angular momentum is conserved is when a disk strikes and adheres to an initially motionless stick nailed at one end to a frictionless surface. After the collision, the two rotate about the nail, and since there is no external torque acting on the system, angular momentum is conserved. On the other hand, a collision in which angular momentum is not conserved is when there is an unbalanced external torque present during the collision. This can occur when an external force is applied to a rotating object, causing its angular momentum to change.