# Conservation of angular momentum in this particular system

A disk of mass M and radius R is located on a frictionless table and pivoted at its center, and initially at rest. A point mass of m with an initial speed v0 hits and scatters from the disk. Since there is no external torque, angular momentum about B is conserved. But why is the angular momentum about A not conserved? I don't see any external torque about there as well. Although I can't prove it with the rules of angular momentum conservation, I know the angular momentum about A is not conserved since $$L_1=0$$ and $$L_2=I\omega$$ thus making $$L_1$$ different from $$L_2$$. Why does this happen? • I'm not sure on this but I think the constraining force that holds the disk at $B$ has a non-zero torque about $A$ Feb 20, 2022 at 10:55
• What makes you think there post-collision rotation. You don't mention friction at all.
– Gert
Feb 20, 2022 at 11:11
• It is said that the table is frictionless and there is angular velocity present for the table post-collision, as shown with ω in the image.
– KayB
Feb 20, 2022 at 11:18
• Sorry but your problem is pretty badly posed. Good luck anyway.
– Gert
Feb 20, 2022 at 11:24 From the diagram you can see that there are impulsive external torques about both $$A$$ and $$B$$ and so angular momentum is not conserved about $$A$$ or $$B$$.