Standing Wave Demonstrator

Description:

It takes a little bit of time to make this demonstration equipment, but it will last for years.

Hold the apparatus by the end of the brightly colored string. Connect the battery so that the motor runs. The apparatus will vibrate causing the string to vibrate. Adjust the length of the string until a standing wave is produced. Changing the length will produce waves with different frequencies and therefore a different number of loops on the standing wave. See how many loops can be produced.

The loops can be viewed with a stroboscope. Holding the apparatus in front of the blue screen on a television screen will cause it to strobe also.

Materials:

Small electric motor (1.5 volts)

AA battery

1.5 length of 3/8 inch dowel

~1 meter length of string or yarn (brightly colored cord shows better)

insulated wire

electrical or other tape

Construction:

  1. Cut the dowel to a length of about 1.25 inches. Drill a hole cross-wise through the dowel closer to one end. The size of the hole should be just smaller than the shaft of the motor.
  2. Press the dowel onto the shaft of the motor.
  3. Feed the yarn through the end of the battery holder where the spring is located. Tie a knot in the yarn, then pull back so the yarn is inside the spring a doesn’t interfere with electrical contact.
  4. Fasten the motor to the battery holder using the double-sided foam tape. The shaft of the motor should extend past the end of the holder so the dowel can turn freely without interference.
  5. Use the leads from the battery holder and solder them to the contacts on the motor. The leads should be shortened so they don’t dangle.
  6. Now wrap electrical tape around the motor and battery holder to secure them. Put a battery in the holder and the motor should turn freely, but create a vibration due to the off-balanced dowel. Unfortunately there is no on-off switch, so the motor must be stopped by grabbing it and removing the battery.


Standing Wave Demonstrator

Teacher Notes

Standing waves are produced by constructive and destructive interference. A wave from a fixed point reflects back 180o out of phase so the incident wave and the reflected wave will have equal wavelength and amplitude If the incident wave and reflected wave are in phase, a standing wave will be set up. The diagram below from ”Hewitt Drew It” clip art illustrates this point. The rope is vibrated by the girl and the rope is the correct length so that the wall is at a node producing a standing wave.

[1]

The situation is exactly the same for the Standing Wave Demonstrator. The vibrations of the Demonstrator produce the waves which are reflected back and forth between the hand and the Demonstrator. Any string length that will cause a node to be set up at the hand (and the Demonstrator) will cause a standing wave. Several different lengths will work producing different numbers of loops.

Adapted from “Teaching About Music”, Clarence Baaken, et al, AAPT 1998

[1] Diagram from clip art Hewitt Drew It ™”, Laserpoint, 1328 W. Palo Alto, Fresno, CA 93711