Date: Fri, 07 Sep 2001 11:05:52 -0500 (CDT)

Author: Dale Stille

Subject: 1D60.20 -- A second opinion



I need a second opinion on demo 1D60.20, the 2nd law apparatus or
Simultaneous fall apparatus. You know, the one where one ball drops
straight down and the other is propelled horizontally and they both hit
the ground at the same time due to gravitational influence. Several years
ago, I took Zig's suggestion and started using 3/4 inch square aluminum
objects instead or the ball bearings so that I wouldn't have to chase them
all over the lecture room.

Now to the problem.....As part of my videocapture upgrade that I am doing,
I have recaptured this demo. The only problem was that although both
square objects appear to hit at the same time when you listen to them,
when videocaptured it is very apparent that the horizontally projected
is about 2 to 5 cm behind when it hits the table. At first I thought it
was just camera positioning and parallax but after correcting and even
overcorrecting for this the results were still the same. After more
careful analysis of the video I think I know where the problem is but
would like some expert opinions to see if I am in the ballpark. My
apparatus is old and has a very small shaft (1/4 inch) which pushes the
object horizontally. No matter how carefully I position the cube, the
shaft always seems to give it a pretty fair rotation rate, usually a
backward rotation even though I try adjusting the height of the object
relative to the shaft. I think that this is due to the fact that the end
of my shaft may not be perfectly perpendicular, but even if it was you
would somehow have to be able to position the object so that it was too.
If I switch back to the round balls, the demo works as it should.

Anyone else try to videocapture this with square object???? What
results?? Anything else that I haven't considered that could be giving me
this result?? ( No, there is not a bump or upward slope to the
object resting platform of the apparatus.)

As usual, any input is useful while I try to do my part to liven the list

Dale Stille
U of Iowa