Conservation of Angular Momentum

[eurekameh]

Why is angular momentum conserved about point A? I know that integral(M,A dt) = H,2A - H,1A, where M,A is the moment of all forces about point A, H,2A is the angular momentum about point A after the impact and H,1A is the angular momentum about point A before the impact. If angular momentum was conserved, it would mean that integral(M,O dt) = 0. In the problem, however, there is a weight that is causing a moment about point A. Why is this ignored?

 

[Doc Al]

In the problem, however, there is a weight that is causing a moment about point A. Why is this ignored?

They are assuming that the collision takes only a very short time--so any effect of gravity can be ignored during the collision. The angular momentum about A immediately before the collision equals the angular momentum about A immediately after the collision.