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Re: Newton's 2nd law Lab



At 10:32 10/12/98 -0500, John Gastineau wrote:
-----Original Message-----

[mailto:PHYS-L@LISTS.NAU.EDU]On Behalf Of Mark Sylvester

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.


If you treat the force sensor like a spring scale, and begin by defining
force
as "what the spring scale (or force sensor) measures" then Mark's first issue
is less significant. The NII experiment then reveals an interesting
correlation between what the force sensor measures and the observed
acceleration.


Yes I agree that the force sensor is essentially a spring scale - it uses a
strain gauge after all. My problem here is with the force units being used
too early, not with the sensor. I think I'm going to try letting the
students calibrate it in arbitrary force units the next time we do this for
N2, so that we can see first proportionality and then introduce the newton
as the unit for which k=1.

Mark has a good second point here, but the accelerometer is a very useful
sensor. One way to make the accelerometer less of a black box is to do a
preliminary experiment using a motion detector and the accelerometer. Measure
the acceleration of a cart on a ramp using both sensors. The kinematical
calculation of acceleration from motion data should be easily understood by
students, and that the accelerometer data corresponds directly will
demonstrate clearly what this new sensor is measuring. This preliminary
experiment need be done only once, and serves as an introduction to numerous
other experiments.


This could only be contemplated if you want to push the line that the
accelerometer is a black box which we see can measure acceleration. If
anyone asks how it does it then the fraud is exposed. In effect, your check
on what the accelerometer measures is the experiment to check Newton 2, so
why do another one? I agree that it's a very useful sensor to have, but for
all the *other* experiments where you want to measure acceleration!

(If you try this experiment, be sure to zero the accelerometer while the cart
is at rest but on the ramp.)

JEG

Mark


Mark Sylvester
United World College of the Adriatic
34013 Duino TS
Italy.
msylvest@spin.it
tel: +39 040 3739 255