A Resonating Idea

Material:

1. one/quarter inch diameter solid aluminum rod about one meter (3 feet) long

2. hard object (small mallet, golf ball, etc)

3. marking pencil

Future Quicktime Movie and photos of the demo to go here!

Procedure:

A. 1. holding the rod between the thumb and forefinger, balance the rod to find the center point.

2. holding the center point between the thumb and forefinger, hit the rod just below this point with a hard object. If a pure resonating tone is not produced, move the thumb and forefinger up or down a short distance and repeat. Do not hold the rod too tightly.

3. place marks on the pipe at 13, 25, 35, and 39 cm on each side away from the center. Holding the rod vertically at each of these points, check each point for resonating tones. Make adjustments as necessary.

DEMONSTRATION:

1. Hold the rod vertically between the thumb and forefinger. Starting at the center mark, strike the rod with the hard object just below the center mark.

2. Repeat while holding at each of the other marks, striking the rod just below the center mark.

3. Repeat while holding one centimeter away from each mark.

You can do the same things by replacing the hard object used for striking the rod, with violin rosin. Coat your thumb and first finger with rosin and while holding the rod as described above, stroke the rod until you set up a standing wave. (You do NOT need to rub the rod very hard. People tend to almost rub their skin off by creating way too much pressure.)

QUESTIONS:

1. What tones do you hear?

2. Why does the rod give different tones?

3. Does holding the rod at different spots determine the pitch?

4. Does the vibration continue if the rod is held at two spots at one time? three spots at a time? Which of the spots?

5. Which spots give the same tones?

6. Which spots on the rod give the highest pitch? The lowest?

7. What is heard when the rod is not held exactly at the specific spots? Why?

RATIONALE:

Striking the rod sets up vibrations in the rod. When held at specific spots, transverse standing waves are created which produce pure resonating tones. The tone/pitch produced depends upon the spots where the pipe is held. The spot 25 cm away from the center gives the lowest pitch, whereas the spots 13 and 39 cm from the center gives the highest pitch due to the shorter wavelength produced. The nodes of the standing waves are located at these specific spots on the rod. When the rod is held at points away from these spots, antinodes are created and the waves interfere or are cancelled out.

APPLICATION:

Musical triangles; musical chimes

RINGING ROD DEMO LESSON PLAN

1. (Option B as a demo write up)

2. For grades 7-12 (physics)

3. Materials: Metal rod about one meter or less in length, and some rosin.

4. Procedure: Find the midpoint of the rod by balancing it on one finger. Hold it in your hand at the midpoint that you just determined. rub the rosin on your other hand between your thumb and your index finger. Now slide your hand down the bottom half of the metal rod with light pressure. The rod will begin to "whine" a screeching sound if done correctly. The more you slide your fingers down it, the louder it gets.

5. Concept: By holding the rod at it's center, you are allowing it to act like a wave. The center is the node of the wave, and both sides resonate as you apply the frictional vibration. By pulling your "sticky" fingers down the rod, you are causing a vibration, and at certain levels you can hear this vibration.

You can tie this demo into a lesson on frequency, wave motion, or sound. You can also obtain a wave with more than one node. By finding the center of one of the halves you have already and holding it there. You now have three nodes. You will need to apply a significant force of vibration, because this frequency is much higher. Any more nodes are usually not going to be heard. You can experiment with different length rods, though.