Dan MacIsaac has a useful article in the January issue of TPT
discussing the operating mechanism of the ubiquituous motion sensor.
Especial useful is a discussion of lab-room difficulties. For
instance, he recommends placing a sock over the sensor when using it
at close range to minimize multiple reflections. I have a few more
practical questions for him or anyone else on this list:
1. The new motion sensors (Pasco II) have both a wide and a narrow
beam setting. When should one use which? So far, I have adopted the
empirical approach: I tell my students to try both and use the one
which seems to work best for them. A typical use is to measure a
rolling cart on a 2-m Pasco track. Curiously, when I go around the
room, I find about half the lab groups swear the narrow beam works
better, and half say the same about the wide beam. Comments?
2. Dan emphasizes that "Advanced methods for calculating less noisy
velocity and acceleration data from sonar position data yield better
data at the expense of a higher student cognitive load and hence are
not defaults in the software." I would like to hear if there is a way
to change the defaults to the "advanced" methods and exactly what
these are. I don't understand the concern about higher cognitive
load, since I don't ask my students to check the finite-difference
calculations. (No flames please. I know, I know: I should have the
students compute these using a slide rule and draw the graphs by
hand, right? :-) Presently, the only "advanced" operation I tell my
students to try is to adjust the sampling rate and the averaging
("Points in Derivative/Tangent Calculations" in Logger Pro --
incidentally, is there a way to average Force Sensor data?) again by
trial and error.
3. Dan recommends adding a "sail" or "flag" reflector to the carts to
improve return signals. I'm a bit worried about air drag - how large
a flag is optimal?
4. Another recommendation is to avoid tennis balls because they are
fuzzy. Indeed, a simple experiment I do early on is have the students
drop a tennis ball onto a motion sensor to determine g and students
often have trouble getting good data. But smoother balls are usually
harder - do folks protect their sensors from impacts? We often use a
grille on a frame boxing the sensor but I suspect these introduce
some noise. (Or students can catch the ball at the last minute but
they are often not coordinated to do this without disturbing the beam
earlier.)
There are probably other good questions about using the sensors. I'd
be interested in hearing tricks and tips from others. Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/