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Re: timing data for falling objects



My sony has a clock with a 1 / 100 s. resolution, so no need for the
computer. Painting meter lines on the side of the building would help.

This reminds me of (most of you have done this) using coffee filters and a
sonic ranger. I've only watched it done, so I don't know how many filters
are necessary to make the time to reach terminal speed conveniently long.

bc


Hugh Haskell wrote:

At 22:33 -0500 7/5/02, Brian Whatcott wrote:

Supposing several seconds to be at least three, and supposing that
represents five stories, then Robert seems to be looking for a log of
one of those 'down the stairwell' data captures, or the video capture
of the World Trade Center collapse in the Phys-L archives.

Maybe I'm being too simplistic, but it seems to me that what Robert
is looking for (Robert can speak for himself better than I, but I'll
toss in my $0.02 worth anyway), is data from a free-falling object
that covers enough time to show the effects of air resistance,
perhaps even terminal velocity. I suspect that it might be difficult
to come by, but how about generating some of your own, by videotaping
an object (a basketball, say, being dropped from the top of a four-
or five-story building. We did that a few years ago and it was
clearly enough to show the effects of air resistance (but not
terminal velocity). It was enough that we could get data on the
coefficient of viscosity of the air. We used Videopoint for the data
collection from the tape.

It's a fairly elaborate project, and takes some planning. The camera
has to be far enough away from the building that the height scale
doesn't change much over the fall, but close enough that you can see
the ball. Using a telephoto lens helps. Then the drop has to be done
while the camera is running (some two-way communications between the
camera crew and the droppers helps. There have to be people stationed
in the drop area to retrieve the ball and return it for another try,
but most importantly to keep passersby from getting bopped on the
head by a rather speedy basketball.

Once you have some good tape segments, the tape has to be digitized
and loaded on the computer where it can be analyzed. With digital
video cameras and Firewire connections between the camera output and
the computer, this isn't much of a problem any more.

It's best to have several runs and let different groups analyze
different sets of data. It shows just how variable data can be,
especially when gathered under field conditions.

What kind of a time constraint are you working under? If my guess
here is correct, I may be able to dig up some of the video we got
when we did this, but everyone is scattered for the summer and I
don't know if I can get hold of the owner of the clips before
sometime next month.

Am I close?

Hugh
--

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

(919) 467-7610

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