Angular momentum demos

# Angular momentum demos

## Angular momentum demos

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##### Presentation Transcript

1. Angular momentum demos Borrowed liberally from John G. Cramer, Professor of Physics cramer@phys.washington.edu

2. Turning a Spinning Wheel Angular momentum is conserved not only in magnitude, but also in direction. It is difficult to reorient a spinning bicycle wheel, so that its spin direction is changed. If this is done in an isolated system, something else must rotate to preserve the conservation of angular momentum about the vertical axis. This effect gives gyroscopes directional stability, which is used in navigation and stabilization devices in ships, planes, and satellites. More on gyros later... Physics 121C - Lecture 22

3. L as a VectorDemonstration: Bike Wheel. I sit on the rotatable stool holding a bicycle wheel that is spinning in the horizontal plane. The rotation axis of the wheel is flipped by 180o. The stool, with me on it, begins to rotate. What’s going on? LW,f Physics 121C - Lecture 22

4. Turning the Bike Wheel Since there are no external torques acting on the person-stool system, angular momentum is conserved. Initial System:LINI = LW,i Final System:LFIN = LW,f +LS LS LW,I LW,i=LW,f+LS LW,F Physics 121C - Lecture 22

5. ?? [1] [2] [3] Clicker Question 1 I am initially at rest on the rotatable chair, holding the wheel spinning as shown in [1]. Now [2] I turn it over and the chair starts to rotate. Question:What happens if [3] the wheel is turned over in the same direction one more time? • rotation stops; (b) rotation doubles; (c) rotation stays the same Physics 121C - Lecture 22

6. [1] [2] [3] Turning the Bike Wheel Twice LNET LNET LNET not turning LW LW LS LW Physics 121C - Lecture 22

7. Support (to be removed) pivot  g Gyroscopic Motion • Suppose you have a spinning gyroscope in the configuration shown below: • If the left support is removed,what will happen?? • The gyroscope does not fall down! Physics 121C - Lecture 22

8. pivot  Gyroscopic Motion • The gyroscope precesses around its pivot axis ! • This rather odd phenomenoncan be understood by usingthe vector character of : Physics 121C - Lecture 22

9. d L  mg Gyroscopic Motion • The magnitude of the torque about the pivot is = mgd. • The direction of this torque at the instant shown is out of the page (using the right hand rule). • The change in angular momentum at the instant shown must also be out of the page! t Physics 121C - Lecture 22

10. d  L pivot  mg Gyroscopic Motion The direction of precession is given by applying the right hand rule to find the direction ofand hence ofdL/dt. Physics 121C - Lecture 22