<-----Original Message-----
<From: phys-l@lists.nau.edu: Forum for Physics Educators
<[mailto:PHYS-L@lists.nau.edu]On Behalf Of David Abineri
<By the way, whenever I have done this form of N2 (cart with wheels
<pulled by hanging mass), most students are invariably confused by the
<question as to, for example, whether the acceleration will be constant.
<They say "of course it will be constant since falling object accelerate
<constantly". I have, therefore tried to avoid the appearance of circular
<logic and tried to avoid using falling masses.
I too have had poor results when doing any kind of Atwood or Half-Atwood
experiment with students, IF the goal is to study NII. The analysis is
invariably circular, and the data collection inevitably tedious so that
students lose track of the point. The data are good, but students don't
understand it.
David's original request was for a good Newton II experiment with an air
track. I've done the following with carts, but this should work much better
with air carts/tracks. Start with an operational definition of force as force
is what a force sensor measures. Compare the force measurements with spring
scales or rubber bands. Once students have a good sense of what the forces
sensor is measuring, go on to explore the accelerometer. Compare its
measurements to timing measurements, if you like, to confirm its operation.
Now you're ready to do NII. Put an accelerometer and a force sensor on your
cart. (If your particular force sensor is too heavy, hold the force sensor in
your hand.) Apply a force to the force sensor which causes an acceleration of
the cart. Plot force vs. time, accel vs. time, and (of course) force vs.
acceleration. No matter what kind of motion is used (quick back and forth,
slow, gentle, whatever) the force and acceleration graphs will be proportional
to one another. The force vs. acceleration graph is a line through the origin.
Neat! Change the mass of the cart, which changes the slope. Cool!
This is a general second law experiment for time-varying forces, which for me
has worked far better than the Atwood experiments I used to do. The Atwood
labs were satisfying to the instructor, but that's about it.
(Thanks, David, for changing the subject.)
JEG
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John E. Gastineau john@gastineau.org
PMB #163 http://gastineau.home.mindspring.com
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