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[Phys-L] Re: Energy & Projectile Lab

The conduit sighting device is a great improvement! I've been having=
the students tape a piece of paper to the floor. They have to figure=
out a "landing zone", using their measurements of the velocity of th=
e ball at the edge of the table. The "zone" consists of three lines t=
hat are drawn on this piece of paper (taped to the floor), parallel t=
o the edge of the table (to allow for side-to-side motion). The cente=
r line is their predicted target, and the other two are an upper/lowe=
r limit based on the variation in their velocity measurements. I'm tr=
ying to get them to think about scientific error properly. Scientific=
error is a NUMBER, not a mistake.
When the students are ready, I make a big deal of placing a sheet of=
carbon paper on top of their target paper. They roll the ball, then=
lift the paper. If they hit their target, they get two extra credit =
points. If they're within the bounds, they get one extra point. To a=
void excessively large bounds, they get one extra credit point (max o=
f three total) if their zone is less than 1 cm wide. Students who thi=
nk that the result was a fluke can then roll the ball several times a=
nd observe the cluster of hits. Some students are amazed by the carb=
on paper - to them, it's a new invention.
The most common mistake (besides not being consistent with their hand=
launching - I don't tell 'em to be consistent, they have to figure t=
hat out) is in measuring the distance between the photogates. We've b=
een putting the photogates about an inch apart at the edge of the tab=
le in order to get an instantaneous velocity, and the distance betwee=
n gates can be difficult to measure well. We don't put the gates far=
ther apart because the ball (a 3/4" steel ball bearing) slows appreci=
ably between the end of the ramp and the edge of the table. We've tri=
ed putting the gates farther apart, but then one measures an average =
velocity that can be considerably less than the instantaneous velocit=
y at the edge of the table. I'm curious, do you avoid=
or account for frictional slowing when the gates are so far apart?


=46rom: Forum for Physics Educators on behalf of Michael Edmiston
Sent: Fri 3/18/2005 12:28 AM
Subject: Re: Energy & Projectile Lab

This is one of my favorite demonstrations. I supervise it, but stude=
do the work. I do it similar to the way Vicki Frohne decribed, with =
added feature that I place a penny on the floor where we predict the
ball will land. I use a one-inch ball bearing, and it leaves a dent =
the penny. I can hit Abraham Lincoln on the cheek every time.

<good stuff snipped>

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
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