How to define change of angular momentum in non-isolated system?

[DannyPhysika]

Homework Statement

http://tycho.physics.wisc.edu/courses/phys201/fall06/Discussion/Disc14Solution.pdf

problem 38, part b)

Homework Equations

I final w final = I initial w initial

The Attempt at a Solution

I understand that the snowball is introducing a net torque and I know that torque = (force applied) x (distance from axis of rotation) (in this case). I am just unsure how the torque was calculated as mvL. I know L is the distance, but isn't mv the momentum of the ball?

 

[ideasrule]

mvL is angular momentum, not torque. Angular momentum is defined as r x p, where r is the position vector and p is the momentum of the object. In this case, r is perpendicular to p, so it makes sense that angular momentum would be mvL.

 

[DannyPhysika]

mvL is angular momentum, not torque. Angular momentum is defined as r x p, where r is the position vector and p is the momentum of the object. In this case, r is perpendicular to p, so it makes sense that angular momentum would be mvL.

Ok that makes sense. If the ball were coming from the right side would we add mvL instead, because the momentum being added to the system is in the same direction of its initial angular momentum?

 

[ideasrule]

Yes, that's correct.

 

[DannyPhysika]

Thanks :)