MBL, was Re: Air resistance

[John Gastineau <gastineau@mindspring.com>]

At 08:49 AM 12/11/97 -0500, Rick T. wrote:
> All of the fussing over these data are precisely the reason I have stayed
> away from 'automated' data acquisition and analysis in introductory labs.

Using MBL (microcomputer based labs) does not rid one of the necessity of
good experiment design. Computer tools can be used badly to acquire
worthless data, just as can manual tools. These examples strike me as
lessons on what not to do, rather than indicating that MBL tools are not
useful with students. 

> One experiment I try every
> year is using a tilted air track to study accelerated motion and in part to
> measure the acceleration due to gravity.  This never works very well (the
> technique requires a photo-gate to be triggered AS SOON AS the cart starts
> to move down the incline and that is difficult to set up properly). 

If the technique has an obvious flaw in requiring a difficult-to-achieve
setup, why use it? There are many other ways to measure g using a
photogate, and they work very well. 

> I
> thought that using a motion sensor, an air track, and a cart with a
> reflector might be the answer.  A couple of us set up the experiment, took
> the data, and tried the analysis.  What we got was GARBAGE.  Ultimately we
> recognized that we did have to average over longer time periods, but even
> then the quality of the data was crap. 

Motion detectors often don't work well with air tracks. The air turbulence
above the track interferes with the desired sound propagation. If you want
to use an air track, photogates are a better bet. 

However, using a cart and a dynamics cart track, you can get very nice data
with little set up trouble. You can easily get velocity and acceleration
data of good quality by tilting a desk, using a cart on the desktop, and
placing the motion detector right on the desk surface. Such data are good
enough to detect the slight difference in acceleration as the cart rolls up
the slope compared to the roll down slope. Good physics, there. 

JEG

 __________________________________

John E. Gastineau      gastineau@mindspring.com               KC8IEW
900 B Ridgeway Ave.    http://gastineau.home.mindspring.com        
Morgantown WV 26505    (304) 296-1966