where v is linear velocity, and r is either (1) the radius of rotation, if the particle is moving in a circle, or
(2) distance of closest approach if the particle is moving in a straight line. (See Figure 16.1 .)
Figure 16.1 Angular momentum.Wait a minute! How can an object moving in a straight line have angular momentum?!? Well, for the
purposes of the AP exam, it suffices just to know that if a particle moves in a straight line, then relative to
some point P not on that line, the particle has an angular momentum. But if you want a slightly more
satisfying—if less precise—explanation, consider this image. You’re standing outside in an open field,
and an airplane passes high overhead. You first see it come over the horizon far in front of you, then it
flies toward you until it’s directly over where you’re standing, and then it keeps flying until it finally
disappears beneath the opposite horizon. Did the plane fly in an arc over your head or in a straight path? It
would be hard for you to tell, right? In other words, when a particle moves in a straight line, an observer
who’s not on that line would think that the particle sort of looked like it were traveling in a circle.
As with linear momentum, angular momentum is conserved in a closed system; that is, when no
external torques act on the objects in the system. Among the most famous examples of conservation of
angular momentum is a satellite’s orbit around a planet. As shown in Figure 16.2 , a satellite will travel in
an elliptical orbit around a planet. This means that the satellite is closer to the planet at some times than at
others.
Figure 16.2 Elliptical orbit.