I use a piece of shelving bracket from Lowe's (or whatever the
local home building supply store is). The kind that you use for
homemade bookshelves (looks like a very shallow U with holes every ha=
lf
inch or so. Comes in lengths from 3 to 8 feet and costs less than a
dollar each. I usually use a 4-foot piece. Buy a piece of cheap 1x2 =
or
1x3 at the same time. Cut the 1x3 into lengths about 6 inches long.
Screw one end of the shelving bracket to the wood piece so that the e=
nd
of the bracket projects about one inch past the wood. =20
=20
Use a C-clamp to clamp the wood piece to the edge of the table s=
o
that the projecting end is out over the floor. The wood piece ensure=
s
that the track end is level and acts to cushion the track. Gently be=
nd
the other free end of the bracket up and support the free end with a
ring stand or whatever. I use one-inch diameter ball bearings as the
roller. The ball bearings will ride on the two "rails" of the shelvin=
g
bracket and will have very low frictional losses. Warning: if the fr=
ee
end is very steep, the ball bearing will slide and not roll at least
initially. When you are done, bend the track straight and store on t=
op
of a cabinet.
=20
I have done this lab two ways. One is to treat the problem as a
full tilt rotational-kinetic energy problem in which case the horizon=
tal
distance that the ball will go is sqrt(10*h1*h2/7) where h1 is the dr=
op
=66rom the start to the track overhang on the table top and h2 is the=
drop
distance from the track to the floor. The horizontal distance is
measured on the floor from directly beneath the overhang to the point=
of
impact.
=20
The other way is to treat this as a mini-lab. I let each table
team (3-4 students) have a set-up and ten-fifteen minutes to run
whatever calculations or experiments that they wish. I then hand eac=
h
team a target photocopied onto a sheet of paper. The innermost circl=
e
is about 3 cm. After time is up, I place a mark on the top end of th=
eir
track with a magic marker. No more experiments are allowed. They ha=
ve
five minutes to tape the target to floor. The ball bearing is placed=
on
mark and released. Where the ball lands is the table's quiz grade fo=
r
the day. In the unlikely event of arguments, I coat the ball bearing
with whiteboard marker and drop it again so that it leaves a mark.
=20
All of which is nice, but what you really want to do is build
yourself a bowling ball ramp and do this experiment seriously. The u=
sed
bowling balls were free from the local bowling alley. The "ramp"
initial was split garden hose glued to two 2x4 rails. Tony Wayne fro=
m
Albemarle High School in Charlottesville, Virginia built one to launc=
h
bowling balls out the window of his classroom. I had one built for m=
e
at Buffalo Gap High School by some of students which operated sort of
like a glorified beach chair. We took measurements of the speed of t=
he
bowling ball at the end of the ramp in our classroom using an early
version of the Pasco motion sensor. We then took the entire apparatu=
s
outside to the football field and launched bowling balls from the roo=
f
of the press box onto the field below. Warning: bowling balls make v=
ery
serious dents, divots when they drop two stories. Another warning:
rolling bowling balls on the second floor of a high school are very
loud. Make friends with the teacher below you.=20