phy 53 lecture dr. Brown 19 october 2010 cross product definition of torque --> definition of anglualr momentum note: \vec{r} measured from a point [not axis] Example: baton rotating at angle about center point -- find angluar momentum real issues with camera note: \vec{L} does not equal I \vec{\omega} L direction is precessing -- ther must be a torque torque = \frac{2 \pi L_{\perp}}{T} = L_{\perp} \omega relate example to balancing tires Angular Momentum orbit of earth about sun sum of orbital angular momentum and spin anglualr momentum = total angular momentum "orbit" of electron about hydrogen nucleus L_orbital and L_spin => these are constant and can be used to "label" via principle quantum numbers another example -- homework problem with hockey puck on frictionless table moving in circle... note expressions for Kinetic energy K = \frac{L^2}{2I} recall K = \frac{p^2}{2m} we used for collisions work = change in kinetic energy example: on turn table -- catch ball [collision] linear momemtum not conservered, nor is kinetic energy -- but angular momentum IS conserved example: sweet spot for collision with rod -- center of percussion