Note: This is a reprint of an article that originally appeared in the LA Times on March 18, 1999.

Last week we talked about how skaters change the speed of their spin. I posed this question to my friend Alicia Alonzo, a graduate student in applied physics. She said that to really understand how skaters spin, you need to know what physicists know about momentum.

??????????All moving objects have momentum. If the movement is in a straight line, it is called linear momentum; or, if the object is spinning, it is called angular momentum. When an ice skater (or rollerblader, for that matter) skates forward and then turns into a spin, they are converting their linear momentum into angular momentum.

??????????One of the cool things about any kind of momentum is that once you have it you can???t easily lose it. For example, when skaters turn into their spin, they don't lose momentum, they just convert it from linear to angular momentum. A physicist would say that this is an example of the Law of the Conservation of Momentum. You can only change your momentum when an extra force gets in your way. For example, the force of friction will change skaters??? momentum slowly, but running into a wall will change it very quickly!

??????????There are several types of motion changes that you can observe to help you think about how momentum is conserved during these changes:

	1. Watch Mother Nature do the twist in ???Hurricanes??? on the Discovery Channel March 21st, at 9pm EST/PST. Ask yourself if there is anything about the behavior of hurricanes which suggests that they conserve their momentum.
	2. Connect to our Web site and try the new Spin Lab simulation. Skaters aren???t the only things that spin! I have several things for you to skewer and spin. Then, think about why some items can be made to go faster than others.
	3. Try to see the conservation of momentum at work using a balloon. Fill a balloon with air and tie the end. The balloon will just stay there forever unless something (like the wind) pushes on it. However, if you open the end of the balloon and let the balloon go, it takes off without anything pushing it. It almost seems like it gains momentum all by itself (which we just said is not possible). So how can this happen? What is countering the forward movement of the balloon so that overall momentum is still conserved?

4. Now see if you can demonstrate the idea of conservation of angular momentum to yourself. Sit on a chair that easily spins. Keep your feet from touching the floor or the chair. Try to quickly twist your upper body around. (It???s not fair to push off with your hands or feet. That???s an extra force!) What happens to the chair? Can you make your upper body move in the same direction as the chair without touching anything with your feet or hands (and without help from a friend!?!). If not, why not??

5. Next week, after I have read your observations from last week (send them in!), we will talk about what the conservation of momentum has to do with figure skaters changing the speed of their spins. (Do you already know? Tell me!)