[Phys-L] Re: measuring the period of a pendulum

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

Larry wrote:

> In an introductory physics laboratory, is it better for students, when
> measuring the period of a simple pendulum, to start and stop the stopwatch
> when the bob is at the lowest point (max. speed) or at an end point (speed
> = 0)?

The standard argument for using the equilibrium position as the
"target phase" for starting and stopping the stopwatch is that, since
the speed is largest there, there is the least uncertainty in WHEN
the pendulum bob passes that point.  It seems to me, however, that it
is at least equally important for the timer to concentrate on the
target phase and then "get into the rhythm" of the pendulum by, say,
moving a hand back and forth in sync with it, perhaps swaying the
body back and forth a little, and counting with an extra verbal "and"
between counts on the return swing before starting the stopwatch and
continuing to do so until stopping the stopwatch.  This helps the
timer to "phase lock" on the pendulum and insure that the starting
and stopping of the stopwatch is done at the SAME phase even if it
isn't very precisely the target phase.

Although I've never done it, I think it might make an interesting lab
exercise to have students determine if different timing techniques
produce significant differences in the UNCERTAINTY of the period
measurement.

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>
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