phy 53 lect dr. brown 14 october 2010 Announcement: will drop 1 quiz for all generalize rotations about any axis -- axis described via vector perpendicular to plane of rotation -- also must communicate orientation [cw or ccw] direction of rotation --> right-hand rule [convention] cross product [review and "know"] note: we use right-handed coordinate system \hat{x} X \hat{y} = \hat{z} \vec{\tau} = \vec{r} X \vec{F} more on definition of cross product of two vectors... using right hand rule properties of cross products Angular Momentum \vec{L} = \vec{r} X \vec{p} --> newton's 2nd law for ang. motion --> \vec{\tau} = \frac{d \vec{L}{dt} ...If total external torque acting on a system of particles equals zero, then the total angular momentum of the system is conserved! ... If \vec{\tau}_{ext} = o , then \vec{L}_{tot} is a constant vector Conservation of Angular Momentum! demos ... MRI's explained ...precession demo'd with bicycle wheel calculated example with wheel as spinning top must know how to do this! [there exists a cleaner calc based method]