What is the conservation of angular momentum?
March 11th, 2007
What do bicycle wheels, figure skaters, and exploding stars have in common? That’s right, angular momentum. This video quotes three others: Rotating Person and Bicycle Wheel, Skating: Scratch Spin, and Exploding Neutron Star.
Transcript:
The Tabletop Explainer
Episode Four (4)
What is the conservation of angular momentum?
Nature conserves spinning. Here a person stands on a turntable capable of turning parallel to the ground. He is holding a spinning wheel perpendicular to the ground and at first he does not spin.
However, when he rotates the wheel, making it spin parallel to the ground, the turntable starts spinning in the opposite direction of the wheel to maintain (that is, conserve) the zero horizontal spinning we started with. As he turns the wheel over, the turntable changes direction again, the wheel’s spinning always canceling out that of the man holding it.
You might notice that the man doesn’t spin as quickly as the wheel. This is because his spinning counts for more because he is more massive. Essentially, there’s more stuff spinning.
However, nature doesn’t only care about mass when spinning is concerned. Distance matters too.
Here a skater brings her arms inward and we notice she turns faster as a result. This is because the large circle spinning counts for more then the small one. So nature in an effort to balance the books, speeds her up.
Scientists call this spinning “thing” angular momentum, and there’s a fixed amount of it in the cosmos. In fact, angular momentum plays an important roll in astronomy, explaining why this binary star collects debris from its neighbor in a rotating disk.
To review, there’s this thing called angular momentum, and it cares about three things.
How fast something’s going
How massive that thing is
How big a circle it traces
Entry Filed under: Conservation Laws
1 Comment Add your own
1. Newtoon | July 27th, 2007 at 10:58 am
OK, this is the “classical” way of explaining things.
Let’s take the skater : I tend to like another explanation relying on the Coriolis effect and I would like to have your opinion on it. To simplify, let’s assume that hands are “weights” and arms have no mass.
Step 1 / the skater has hands far from her body and hands are going fast in linear speed (at instant “t”).
Step 2 / the skater takes hands back to the body. The linear speed
of hands tend to be slowed down more and more when the skater take the hands back to the body BECAUSE of the efforts of the skater (otherwise the right arm would to go toward the front of him and so forth and the left arm would tend to go toward the skater’s back / if no efforts are applied)
Step 3/ Application of Third Newton law : there must be a reaction to these efforts and this is the acceleration in rotation speed.
PS : I talked about “efforts” of the skater but this would work anyway with cables and weights (thanks to cable tension).
If you imagine that the angular speed of the vertical axle is fixed (e.g. by a “stubborn” motor), when diminishing the radius, the weights and cable would wind up around the axle !
Was I clear ? What do you think of the idea please ?
This explanation has the advantage to not use the term “Angular momentum” and replace it with Coriolis. Where is the point ? I feel that Coriolis is easy to explain without equations or theories of anykind.
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