Materials:
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- one/quarter inch diameter solid aluminum rod about one meter (3 feet) long
- hard object (small mallet, golf ball, etc)
- marking pencil
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| Directions: |
- Holding the rod between the thumb and forefinger, balance the rod to find the center
point. 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.
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- 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. 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.)
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| Safety Concerns: |
- None.
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| Questions: |
- What tones do you hear?
- Why does the rod give different tones?
- Does holding the rod at different spots determine the pitch?
- Does the vibration continue if the rod is held at two spots at one time? three spots at
a time?
- Which spots give the same tones?
- Which spots on the rod give the highest pitch? The lowest?
- What is heard when the rod is not held at the "correct" spots? Why?
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| Concepts: |
- 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 give 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.
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