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Re: Air resistance




It is conceivable that air turbulence causes the problem, but it doesn't
seem that likely to me.

As to the air track problems, Jim Riley posts that he uses an air track
with no problem at all. That may be; air tracks are different from one
another, and so some may exhibit the problem more than others. Some air
tracks just don't mix well with the motion detector. Others do fine.

Motion Detector use is not quite as simple as it seems at first. As others
have said, you can often go from garbage data to quite good data by
adjusting the angle of the detector by a few degrees. A coffee filter isn't
much of a target, nor are styrofoam or foam balls. You can get a
reflection, but expect some noise. The stronger echoes from harder targets
generally give better, quieter data.

Kyle correctly points out that many student experiments aren't don't give
extremely high quality data. The motion detector really isn't designed to
give data of better than a couple of mm resolution; its original use was
with qualitative graphing. It does work pretty well with quantitative
work, too, but if pushed too far, it'll yield nonsense. It's just like any
other measurement.

As an example, everyone wants to measure g. The motion detector does a
great job of measuring accelerations of a cart on a ramp, for example, but
in measuring g directly it only does a fair job. The experiment is on the
ragged edge of what the MD can do. That doesn't mean the MD is a bad tool;
it just means that it's better at cart-on-a-ramp accelerations than freefall.

JEG




At 01:34 PM 12/11/97 -0400, you wrote:
Date: Thu, 11 Dec 1997 11:12:01 -0500
From: John Gastineau <gastineau@mindspring.com>
Subject: MBL, was Re: Air resistance

Motion detectors often don't work well with air tracks. The air turbulence
above the track interferes with the desired sound propagation.

Is it conceivable that the probing waves are reflected from turbulances
created by the folling ball (or filter) rather than from a solid surface?
This can explain our occasional "something was wrong" manifestations when
the object was monitored from above? Perhaps monitoring from below, rather
than from above, will help to eliminate sporadic bad data. The instruction
manual should tell us about such things.
Ludwik Kowalski



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John E. Gastineau gastineau@mindspring.com KC8IEW
900 B Ridgeway Ave. http://gastineau.home.mindspring.com
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