Sunday, January 6, 2013

How to do Conservation of Angular Momentum in Inelastic Collisions

This video goes through the setup for two examples of Inelastic collisions that involve rotations.  One is a sky diver landing on a spinning merry-go-round (disk) at some angle, and the sky diver 'sticks' to the platform.  If there is a component of the sky diver's motion that is perpendicular to the rotational motion of the disk, then the rotational motion will change.  Angular momentum is conserved for the system if there is no external torque acting on the system, which is the case here.  So we use the definition L = Iw for the disk since it is actually rotating, and L = mvrsin(angle between the line of motion and radius from axis of rotation).  Again, like torque, we are looking for the component of motion perpendicular to the radius line, since that is the component that can affect the rotation of the system.

The second example is a particle moving linearly running into and sticking to a stick that is nailed down at its center. After the collision, the system only spins, so we have both L = Iw and L = mvrsin(angle).  See if the setups make sense.