Lecture 15 Angular Momentum l Definition of Angular

Lecture 15: Angular Momentum l Definition of Angular Momentum l Right Hand Rule l Conservation of Angular Momentum Example

Define Angular Momentum p = m. V L = I conserved if Fext = 0 conserved if ext =0 Vector! units: kg-m/s units: kg-m 2/s

Right Hand Rule l In general, for an object rotating about a fixed (z) axis we can write LZ = I . l The direction of LZ is + CCW, - CW. z èWrap fingers of right hand, around direction of rotation, thumb points in direction of L.

Ice Skater Example l An ice skater is spinning in a circle at 4 rad/s. He wants to increase this rate, so he pulls in his arms to decrease his moment of inertia from 3 kg-m 2 to 0. 8 kg-m 2. What is his new angular speed? èWith no (or negligible) external torque acting, we can use Conservation of Angular Momentum: Li = L f

Ice Skater Example l An ice skater is spinning in a circle at 4 rad/s. He wants to increase this rate, so he pulls in his arms to decrease his moment of inertia from 3 kg-m 2 to 0. 8 kg-m 2. What is his new angular speed? èSubstitute in the known values and solve: Li = L f I i i = I f f (3 kg-m 2)(4 rad/s) = (0. 8 kg-m 2) f f = 15 rad/s

Summary l. L =I èRight Hand Rule gives direction èIf = 0, L is conserved