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Re: [Phys-l] projectile motion lab




----- Original Message ----- From: "Hugh Haskell" <hhaskell@mindspring.com>

Photogates will likely have a rather larger gap between the sending
and receiving sides that the size of the object passing through them,
and so the actual time of starting or stopping the timer can be
affected by the position of the ball as it passes through any gate,
closer to one side or the other. If one looks at the actual signal
that passes between the arms of a photogate as an object passes
through, it will be seen that the signal does not go suddenly to zero
but decreases at a more moderate pace. The choice of the point in the
signal roll-off where the time actually shuts off, is the source of
most of the imprecision in photogate timing, and the actual shape of
this curve is surprisingly sensitive to the shape and position
relative to the arms of the object passing through.


I second this--and in fact caution that depending on your equipment, there can be delays of several milliseconds in the turn on/off timing. I ended up junking our Thornton photogates and timing systems because of this--and the fact that different gates produced different delays for the same timing. We moved to the Daedalon photogates and times for airtrack work and since then have moved to the Pasco photogates and the Smart Timers. So, if you need millisecond timing and reproducibility, you need to test your equipment to see if you really have it.

Rick