文档介绍：Angular Momentum Conservation where and l l In the absence of external torques In the absence of external torques Total angular momentum is conserved. Total angular momentum is conserved. (demos) (demos) ??F EXT? d ?pdt ??? EXT??r??F EXT ??? EXT? d ?Ldt ?0 1/13/2017 1 Phys 201, 2011 I iω i = I fω f 1/13/2017 2 Phys 201, 2011 Spinning skater: Because the torque exerted by the ice is small, the angular momentum of the skater is approximately constant. When she reduces her moment of inertia by drawing in her arms, her angular speed increases. 1/13/2017 3 Phys 201, 2011 A student sitting on a stool that rests on a turntable with frictionless bearings is holding a rapidly spinning bicycle Wheel (a). The rotation axis of the wheel is initially horizontal, and the magnitude of the spin-angular-momentum vector of the spinning wheel is What will happen if the student suddenly tips the axle of the wheel (b) so that after the rotation the spin axis of the wheel is vertical and the wheel is spinning counterclockwise (when viewed from above)? Answer: The turntable, stool, and student will be rotating clockwise with an angular momentum about the vertical axis of the turntable of magnitude Example: Two Disks ? A disk of mass M and radius R rotates around the z axis with angular velocity ??? i . A second identical disk, initially not rotating, is dropped on top of the first. There is friction between the disks, and eventually they rotate together with angular velocity . ? What is the relation ship between and ????????? ? analogous to an inelastic collision: p 1 + p 2 (=0) ? p 3 zz ? f?? i? i? f ? f 1/13/2017 4 Phys 201, 2011 Example: Two Disks ? First realize that there are no external torques acting on the two-disk system bined mass M. – Angular momentum will be conserved! ? Initially, the total angular momentum is due on