# Conceptual Q: Angular Momentum

1. Mar 24, 2013

### tolove

L = r x p

What's the significance of angular momentum, L, acting along the z axis? Angular momentum is being compared to translational momentum, p = mv, but translational momentum is acting the same direction of the velocity, right?

Thanks for any help!

edit: this is a nice gif:

http://en.wikipedia.org/wiki/File:Torque_animation.gif

2. Mar 24, 2013

### Staff: Mentor

Angular momentum along the z axis means that the object is rotating about the z axis. The velocity of the particles is perpendicular to the axis of rotation, hence L=rxp.

3. Mar 24, 2013

### tolove

I think I understand what you're saying. Just hit a section about precessional motion. I think this will fill in the blanks when I get through it.

Closing my book for tonight I think~

4. Mar 25, 2013

### tolove

Alright, not sure I completely understand this concept yet. Could you answers these questions for me?

A dreidel is spinning away, no resistance from air or friction from surface (or anywhere else). It mas mass, angular speed, etc. Nothing special about it.

1) The dreidel is placed perfectly perpendicular on top of a horizontal surface. If there is no gravity, will the dreidel move in any way or eventually come to a stop? Of course not, right? The graphs of L acting on z were confusing me, though.

2) The dreidel is placed at a slight angle away from perpendicular on top of a horizontal surface, under gravity. Will the dreidel eventually come to a stop?