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

2. December 5, 2005Physics 121C - Lecture 222 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...

3. December 5, 2005Physics 121C - Lecture 223 L as a Vector Demonstration: 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 180 o. The stool, with me on it, begins to rotate. What’s going on? L L W,f

4. December 5, 2005Physics 121C - Lecture 224 Turning the Bike Wheel Since there are no external torques acting on the person-stool system, angular momentum is conserved. LL Initial System: L INI = L W,i LLL Final System: L FIN = L W,f + L S L L W,F LLSLLS L L W,I LLL L W,i = L W,f + L S

5. December 5, 2005Physics 121C - Lecture 225 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? (a) rotation stops; (b) rotation doubles; (c) rotation stays the same [1] [2] ?? [3]

6. December 5, 2005Physics 121C - Lecture 226 Turning the Bike Wheel Twice L L NET LLWLLW LLSLLS LLWLLW not turning LLWLLW [1] [2] [3]

7. December 5, 2005Physics 121C - Lecture 227 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!  pivot Support (to be removed) g

8. December 5, 2005Physics 121C - Lecture 228 Gyroscopic Motion precesses The gyroscope precesses around its pivot axis ! This rather odd phenomenon can be understood by using the vector character of :  pivot

9. December 5, 2005Physics 121C - Lecture 229 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!  L d mg t

10. December 5, 2005Physics 121C - Lecture 2210  L The direction of precession is given by applying the right hand rule to find the direction of  and hence of dL/dt. Gyroscopic Motion  L pivot d mg 