2007 AAPT Summer Meeting
Greensburo, NC
(Standing Waves, OHP)
Doug
Johnson, Maria Salazar, Anatol Hoemke, Hector Maciel
California Polytechnic University,
Pomona
3801 Temple Ave
Pomona, Ca 91768
909-869-4035
hmmaciel@csupomona.edu, dcjohnson@csupomona.edu, mariasalazar@csupomona.edu, ashoemke@csupomona.edu
Abstract
This
demonstration shows standing waves on a string. By varying the tension on the string, we can show the
fundamental frequency, first harmonic, second harmonic, overtones, nodes and
antinodes.
Construction of Apparatus:
Materials Needed:
Lumber-
3/4"
Plywood, cut @ 11"x 20"
2x4 (Real
dimensions 1.5"x 3.5"), cut as follows:
2 @ 9"x 1
5/8"x 1.5"
1 @ 6.5"x
3"x 1.5"
1 @ 3"x
1.5"x 1.5"
1 @ 3 3/16"
round x 1.5" height
Screws-
4 @ 1 3/4"
flat head wood screws
2 @ 1 3/4"
pan head wood screws
2 @ 1 1/2"
pan head wood screws
6-12 @ 3/4"
flat head wood screws
Other-
Cenco string
vibrator
1" length 16
AWG wire
PVC
3"X3" Coupling hub
3 Corner brackets
Screen door
tension roller (w/ screw)
2"x 1"
Felt strip
18" length
rubber string (from a paddle-ball toy)
Cable bracket (w/
nail)
Primer and spray
paint (optional)
Overhead
projector
Approximate
Building Materials Cost:
$10, not
including the Cenco vibrator and paint
Tools Needed:
Soldering iron
and solder
Table saw
Radial saw
Sander and
sandpaper
Jig saw (for the
plywood piece center cut-out)
Power drill with
various drill bits, phillips and flat-head driver bits, countersink bit
2 Clamps
Pliers
Awl
Masking tape
Assembly
Instructions:
1. Make a small loop with the 16 AWG wire
and solder it to the tip of the vibrating reed on the Cenco vibrator as
shown. The holes already present
are too small for the use of this project and can cut the rubber string we will
attach to the end.
2. Cut the plywood and 2x4 to the required
dimensions (as listed above). Mark
the layout of the center cut-out as shown in the diagram and drill the corners
with a 3/8" bit. Use the jig
saw to cut the center out of the plywood piece.
3. Lay the plywood over the 9" blocks
with them centered on the short sides of the plywood. Those are the legs that will act as a rail to keep the piece
from sliding off the overhead projector.
Clamp them in place firmly and drill two holes for each rail from the
top face of the plywood into the rails for the 1 3/4" flat head wood
screws. Countersink the holes with
a countersink bit or a standard 3/8" bit. Screw the pieces together with the 1 3/4" flat head
screws.
4. Place the other wooden pieces on the
board in their designated positions as diagrammed. Mark all the exposed edges and corners of all the pieces,
and sand them. If desired, paint
all the pieces.
5. Center the Cenco vibrator on the top of
the 6.5" block as shown and mark the two middle mounting holes with an
awl. Drill them and attach the
vibrator with the 1 1/2" pan head wood screws.
6. Drill two holes approximately centered
and 1" apart completely through the 3 3/16" round wood piece from the
top face. Attach the round piece
to the plywood as diagrammed - the edge of the round wood piece should be about
1 1/8" away from the bottom and the side of the plywood, so that when the
PVC piece fits over it the PVC is about 1" away from the bottom and side
of the plywood. Place the felt
strip against the inside of the PVC coupling and slide it over the round wood
piece. The felt should give enough
friction so that the PVC turns smoothly yet is somewhat tight.
7. Using the pliers, bend the screen door
tension roller slightly at it's midpoint as shown. Using the screw included with the tension roller, attach the
roller 1 1/2" out from the unsanded side of the 3"x 1.5"x
1.5" wood piece, as shown.
8. Using the masking tape, secure the
blocks in position as diagrammed.
Place the corner brackets in their positions as shown and mark the
center of the screw holes with an awl.
Remove the blocks and brackets and drill shallow pilot holes in all the
marked spots. Using the 3/4"
screws, attach the brackets to the blocks and the plywood base. Tie one end of the rubber string to the
Cenco vibrator, and tape the other end to the PVC. Run the rubber string over the wheel on the tension roller
and turn the PVC to increase/decrease tension.
9. Throw it on the overhead projector,
plug it in, and see how many standing waves you can get!
Use of Apparatus:
Standing wave apparatus should be place on a
overhead projector, so image can be displayed on white screen. Plug-in vibrator and adjust tension on
string to harmonic of your preference.