Re: Newton's 2nd law Lab

[Mark Sylvester <msylvest@SPIN.IT>]

I have some reservations about the mbl experiments for Newton's second law,
and wonder if they are shared.

Minor reservation: I don't like using a force sensor already calibrated in
newtons when we want to show the second law.

Bigger reservation: I don't like using an accelerometer which works by
inertia for this experiment. If we treat the sensors strictly as black boxes
this may be OK, but I like the students to have some idea of what the sensor
does.

The force sensor is OK since it works by stretching, and the difficulty is
with the unit only, but I much prefer getting the acceleration
kinematically. Yes, the data can be messy if you "double difference" the
position measurements, but it's easy and quick enough to do straight line
fits to v-t graphs for a series of constant forces, and the graph is not too
bad for the experiment that John Gastineau describes below.

Mark

At 08:56 10-12-98 +0100, John Gastineau wrote:

...discussion of half-Atwood experiment deleted...

> > A much cleaner experiment is possible using a force sensor and an
> > accelerometer. Attach both devices to a cart; move the cart by hand using
> only
> > the force sensor. Record both the force vs. time and acceleration vs. time
> > data, then plot force vs. acceleration. The points should be on a line with
> > slope m, where m is the total mass of the cart, accelerometer and force
> > sensor. Now change mass of the cart. New slope.
> >
> > This experiment is much, much cleaner than the old half-Atwood method for
> > doing Newton II. The half-Atwood approach done with MBL is just a
> computerized
> > version of a non-computer method.
> >
> > You can also do this experiment with a motion detector and force sensor, but
> > the data quality suffers considerably since you must work with a second
> > derivative to get the acceleration data, while the accelerometer gives it to
> > you directly.
> >
> > I can point you to more details if needed.
> >
> > JEG



Mark Sylvester
UWC of the Adriatic
34013 Duino TS
Italy
msylvest@spin.it