Re: Newton's Second Law Lab

[Mark Sylvester <msylvest@SPIN.IT>]

At 08:13 PM 7/4/00 -0400, Bob Sciamanda wrote:
> ----- Original Message -----
> From: "Ludwik Kowalski" <KowalskiL@MAIL.MONTCLAIR.EDU>
> To: <PHYS-L@lists.nau.edu>
> Sent: Tuesday, July 04, 2000 6:11 PM
> Subject: Re: Newton's Second Law Lab
> > , , ,
> > On the same screen you see three independent graphs.
> > One is above, showing F(t); one is below showing a(t).
> > Both are measured independently (several times each
> > second) as you are pushing and pulling the platform
> > back and forth through a forcemeter).
>
> To be sure these "F" and "a" measurements are truly independent and direct
> measurements of force and acceleration, one should inquire about the
> mechanisms of the measuring devices.
> Eg., is the accelerometer directly measuring a kinematical quantity, or a
> dynamical quantity?


I raised this point the last time we went round this issue (a year or so
ago?). The opinion was expressed then that it was pedagogically OK for the
accelerometer to be a black box that measures acceleration. Of course, it
and the force sensor both work by distortion, so they're the same device,
placed back to back. But the experiment works also if you use the position
sensor and calculate acceleration by double differencing. It's interesting
then to see the difference in the graph. I find it's important to do all
this with the IB classes that I teach since it makes the difference between
the datalogging toys being a measurement tool as opposed to just a magic show.

For years we did this stuff by pulling wooden carts with elastic strings.
Great for kinesthetics, but the graphs, which took some work to produce,
needed imagination, not say an act of faith, to interpret. Is this where
the old "if it doesn't work, it's physics" begins?

Mark