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*From*: Daniel Crowe <dcrowe@SOTC.ORG>*Date*: Fri, 18 Mar 2005 19:05:58 -0700

I regularly have students measure the effective lengths of air-track-

glider flags in photogates. It should be possible to measure the

effective length of a ball in a photogate. You need to measure the

positions at which the ball first triggers the photogate and last triggers

the photogate. This can be determined by observing the LED on top of the

photogate. Students who rush through this procedure often have large

relative errors in their lab results, but students who carefully measure

the effective lengths have much better accuracy. Using the diameter of

the ball would definitely lead to large errors.

Daniel Crowe

Oklahoma School of Science and Mathematics

Ardmore Regional Center

dcrowe@sotc.org

On Fri, 18 Mar 2005 18:21:45 -0500, Michael Edmiston

<edmiston@BLUFFTON.EDU> wrote:

<snip>

I have heard of "failures" of the experiment when the teacher uses one

photogate and tries to use the diameter of the ball as delta-x. This is

tough with a round ball because you cannot guarantee the center of the

ball will go through the beam, plus photo gates suffer from a sort of

parallax that makes objects appear thinner than they really are. So

don't use one gate.

<snip>

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