Re: [Phys-l] projectile motion lab

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

> > 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.  

I don't think the standard deviation of the mean (aka "standard error")
is what is needed here.  The st. dev. of the mean tells you how well you
know the mean value, but that isn't what is important.  You want the
effect of the variation of the individual trials, which is related to
the plain old standard deviation.  The standard deviation is related to
how close the center of the trials would be to the "true" value.

It does sound like the predicted uncertainty is much to large given the
numbers you have, but of course more detail would be needed to give a
more definitive answer.


Tim F