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Mar 18,
1999 |
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 By Dr. Leila
Gonzalez (and friends)
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.
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 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!
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There
are several types of motion changes that you can observe to help you
think about how momentum is conserved during these changes:
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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.
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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.
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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?
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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??
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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!) |
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