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Activity
12. Plucked cords and rubber bands
Equipment needed:
- Vivitar 283 flash unit
- 4 AA batteries or SB-4 AC adapter
- Cord to connect flash unit to delay circuit
- Flash clamp
- Delay circuit
- Two 0.01-mf capacitors
- Bracket photogate and trigger circuit (transistor type)
- Cord from photogate trigger to delay circuit
- Elastic cord
- Two supports for the cord
- Rubber bands
 Background:
In Activity 9, you used an
interrupter photogate to observe waveforms on a plucked, elastic
cord. You achieved different delays by moving the interrupter to
various positions along the cord. In this activity, you’ll use a
bracket-type photogate in a fixed position in order to obtain a
wider range of delays. The bracket, shown to the right, holds the
infrared LED and photodetector farther apart than the interrupter,
thus allowing larger objects to pass through.
- In order to observe waveforms on an elastic cord, connect the
output of the photogate to the input of the delay circuit. Use a
0.01-µf capacitor with the delay unit, and connect the flash
unit to output 2. Adjust the photogate to maximum sensitivity as
described in Activity 9.
Set up the arrangement shown below. Tape the photogate to the
tabletop and stretch the cord between two fixed supports. Pull
the cord down into the bracket and release it in order to
discharge the flash unit. Adjust the delay in order to show the
cord at different times after release. If you need greater delay
times, use a larger capacitor. Adding a second 0.01-µf
capacitor in parallel with the first will double the maximum
delay time.
- Try shooting a rubber band through the photogate. Adjust the delay to show the band at various times after it
leaves the gun. In order to get reproducible results, you'll
need to shoot the rubber band in a similar way each time. You
may want to make a gun using a board and clothespin.
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