Re: [Phys-l] projectile motion lab

["Krishna Chowdary" <professor.head@gmail.com>]

On 10/9/06, fizix29@aol.com <fizix29@aol.com> wrote:
> I used a student's sample of about 50 time measurements, that is, the time
> it took the marble to roll between the two photogates.  I used the mean time
> and the distance between the two photogates to calculate the speed of the
> marble as it rolled off.  I used the standard deviation of the time
> measurements as the error in the time and a reading error of +/- .5 mm for
> the distance measurement.


I'm interested.  What is the mean time and the standard deviation using this
data set?  Also, I suspect that you should use the standard deviation of the
mean as your uncertainty.  In your case, that will reduce your uncertainty
by the square root of 50, or by a little more than a factor of 7(!), which
seems pretty substantial to me and might be a problem with your error
analysis.


> When I carried the calculations through (I also measured the height of the
> table to +/- .5 mm) I found that the range of the projectile was only
> certain to about 30%.  It could be as low as 20 or as high as 40 cm.


Could you describe how you did the calculations, specifically the error
propogation?



> That hardly seems a triumph for newtonian mechanics!  There must be a flaw
> in my assumptions.  I confess I am only now beginning to include rigorous
> treatments of error and uncertainty in my labs.  It was a fault of my youth
> I am trying to correct!

Good for you!  An understanding and appreciation of sources of experimental
uncertainty, some methods for describing uncertainty quantitatively, and
some tools for propogating uncertainty through a calculation should be among
the key learning outcomes of a physics lab course, in my opinion.

--
regards
-Krishna

Krishna Chowdary
Department of Physics & Astronomy
Bucknell University