I second John's suggestion of Small Parts Inc. I have bought a lot of
stuff from them. Service is good. They have items that are difficult
to find elsewhere. Prices are reasonable. You could beat the prices
for some things by buying large quantities from another source, but
that's not the purpose of Small Parts Inc. They will sell many items in
quantities of one.
Vicke asked about my spacing of the photogates so far apart. I have
found that I do have to be careful about my choice of table. If the
table is not level the ball could either speed up or slow down between
the ramp and the end of the table. As I reflect more on what I have
done in the past, I think we time 0.5 m or up to 1.0 m when we use stop
watches; which we sometimes do. When we use photogate timers I think we
time 0.2 to 0.3 m.
As delta-x gets too short you get increased error because the round ball
going through the beam does not necessarily trigger the gate at the same
relative position between the gate and ball for each gate. If delta-x
is too large, then Vicke is correct that we don't have an instantaneous
velocity if the speed of the ball is indeed changing. For a hard level
table, a 1-inch ball bearing does not change speed very much. A
non-level table is a different story.
I have heard of "failures" of the experiment when the teacher uses one
photogate and tries to use the diameter of the ball as delta-x. This is
tough with a round ball because you cannot guarantee the center of the
ball will go through the beam, plus photo gates suffer from a sort of
parallax that makes objects appear thinner than they really are. So
don't use one gate.
Dwight says he does not have photogate timers. Digital wrist watches
with stop-watch function that display to 0.01s work fine if you time a
large enough delta-x and do it many times. If students start the watch
when the ball hits the table as it exits the ramp, and stop the watch
when it hits a block of wood near the edge of the table, and these are
0.5 to 1.0 m apart, students can get consistent timing after a bit of
practice. With several watches, several students can time each event
and compare/average their results. This does indeed require a level
table.
Concerning carbon paper... my supply was running low and the campus
bookstore had none. I sent an e-mail to all faculty asking them to go
through their desks and filing cabinets and send me any carbon paper
lurking there. From a faculty of about 100 I received about 15 boxes of
carbon paper, some unopened. I ended up with over 1000 sheets. The lab
I use it for is a projectile motion lab where the ball from a launcher
hits a vertical board with paper and carbon paper taped to it. Students
place that at various positions downrange from the launcher and find the
height at that range. It's similar to using the hoops. They put the
data into Excel and fit a parabola to it. It works well. Each year I
use about 5 sheets of carbon paper. I have enough to leave for several
generations of professors after I am gone.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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