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Re: Lift of spinning objects.



I always demonstrate this phenomenon using a Wham-O toy called, I think,
"Trak Ball". It is in principle like a jai-alai cesta and pelota, but it
uses a clever plastic cage with a serrated track and a larger ball of
styrofoam. It requires no skill to throw inverted curves using this
device. I always bring it out when I talk about parabolic trajectories
(yeah, I like jokes) but I never try to explain its physics - or
airplane lift, either. I find Hewitt style explanations of these
phenomena conceptually vacant. I have another version of this toy, a
Taiwanese ripoff called "Trail-Ball" at home. It's a great toy which can
be used to amuse children, as well.

(No, I don't even know if Hewitt treats lift, but I'm prepared to doubt
that he does so valuably. No other elementary textbook I've seen does
any better.)

Leigh

Bulls-eye on this one, Leigh. Hewitt does (or did, in an earlier
edition--I haven't checked to see if he has corrected it) have a
pretty botched up explanation of lift, in which he separates out the
"Bernoulli" lift, and the "impact lift," and goes so far as to claim
that the "Bernoulli lift" does not involve a reaction force anywhere.

It was written up in Mario Iona's column in TPT some years back.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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