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Re: A Parents' Day gem



"That's LUDICROUS!" said the parent. "ANY fool can see that the forward
momentum of the SHOT bullet will make it spend LONGER in the air that the
DROPPED bullet!"
"I can prove it to you mathematically if you wish," replied the teacher,
"but they will, in fact, hit the ground at the same time."
A pause. Then:
"This is witchcraft!" the parent hollered. "WITCHCRAFT!!!" And he
turned
and stormed out of the classroom.


I've done the experiment with a gun--sort of--to convince a companion.
Pellet rifle shot over a calm lake, another pellet dropped vertically. The
splashes were, at least, very close in time. Of course, we had no way to
accurately level the gun; I tried to hold it level but suspect I
unconciously changed the angle until I got good results.

I did mention things like curvature and so on to my friend. He did not
follow all of that but did decide that his original assumption that a large
forward speed would cause a much larger flight-time was not correct.

Robert Mathieson
Culver-Stockton College
Canton, MO 63435
(217)233-6000
rmathieson@culver.edu

It sounds like this was a rather informal "experiment." So not only was the
leveling of the gun barrel only approximate, but how did you insure that
the drop and shoot times were about the same and how did you determine that
the landing times were about the same? The two events had to have been far
enough apart that sound was not a reliable indicator. Unless both landing
points were in the simultaneous view of at least one observer (that is to
say, approximately colinear as seen by that observer), and the splashes (I
assume the shot was fired over water so the dropping bullet would land in
the water and splash like the fired one) were both large enough to see, it
seems to be quite difficult to establish the simultaneity visually.

While in principle this experiment seems simple, I believe it is really
hard to do in the real world, even approximately. This is an example of a
principle that I have found through repeated and sometimes embarrassing
experience. When trying to convince the unconvinced of something that we
take almost as a given, the experiment has to be chosen so that even when
done badly, the results turn out right. In these situations, it doesn't
work to try to talk your way out of a poorly designed experiment by saying
"it works if I do it better," or words to that effect. And it is wrong to
secretly skew the experiment so that it appears to come out "right" even
when it hasn't. Unfortunately, experiments that satisfy the criterion that
they work even when poorly done are very hard to find or to design.
Hovever, the simple apparatus that will launch a aprojectile while
simultaneously dropping another one are easy to devise and work quite
reliably, either the meter stick one described by Ludwik, or one of the
simple kits available from Pasco or other suppliers, or any of several
other designs, all seem to fit the required criterion, at least within the
confines of a few meters. Usually that is enough to convince those like the
irate parent, who, as others have pointed out, seemed unlikely to have been
sophisitcated enough to understand the subtleties of projectile motion
through a dissipative atmosphere, over a curved, rotating surface.

Hugh

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

The box said "Requires Windows 95 or better." So I bought a Macintosh.
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