# Angular Momentum

## Program 119

An old momentum with a new twist. Kepler's
second law of planetary motion, which is rooted here in a much deeper principle,
imagined a line from the sun to a planet that sweeps out equal areas in equal
times. Angular momentum is a twist on momentum -- the cross product of the
radius vector and momentum. A force with twist is torque. When no torque acts on
a system, the angular momentum of the system is conserved.