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Integrated Coordinated Science for the 21st Century

Active Physics
+ Chapter 5
Long-Distance Communication
Answers
Physics To Go
  1. a) 0.5 waves per second.
    b) 2.5 m/s
  2. a) The pulse turned upside down.
    b) A water wave would not turn upside down.
    c-d) No, it reflects the opposite way (it does not invert).
  3. When a water wave goes by, the water moves above and below its normal position. When the wave has passed, the water returns to its normal position (a horizontal surface). The spring behaves the same way.
  4. Both transmit speech. The cup phone transmits the vibrations of speech directly. The telephone converts sound to electricity and transmits the electricity. At the other end of the connection, the electricity is converted back to speech. The telephone has the better sound quality.
  5. If you made the cup very large, some of the sound would escape around the rim of the cup. On the other hand, a large cup would have a large bottom to vibrate the air more. If you made the cup very small, the small bottom would be unable to vibrate the air very much, so there would be little sound. You could test the prediction by making cup phones with different sized cups.
  6. The cup phone is similar to the washtub base because in both, a string vibrates the base, which vibrates the air, just like a speaker. In the cup phones, the cups act as both microphone and speaker.
  7. If the cup phone code is based on plucking the string, one advantage is the clear, distinct nature of the sound. One advantage of a Slinky code is that it can be observed by both looking and by listening. For each, the code can provide privacy.
  8. Variables that affect the speed include the Slinky tension, the material of the Slinky, the thickness of the coils, and the geometry of the coils.
  9. It may be apparent because the loudness of the transmitted sound may decrease or because of the extra pull in the string. Maintaining a clear line of sight along the string can prevent this kind of eavesdropping.
  10. a) There is no way to tell.
    b) Answers will vary.
  11. Perhaps there is a special radio signal for each letter.