Workshop module 12 - Physics 113, Fall 2000
  1. Why do you see lightning before you hear the thunder? Would you see a rocket lift off from the moon before you heard it? You see a baseball player swing at a ball 1.0 seconds before you hear the crack of the bat on the ball. How far away from home plate are you sitting?
  2. In 1989, minutes before the start of the first game of the World Series in Candlestick Park, an earthquake struck the San Francisco area. This quake has become known as the Loma Prieta quake. It was my first … and hopefully last … experience with earthquakes.

    3. (a) What is an earthquake?

    (b) What are seismic waves?

    Longitudinal seismic waves are known as P waves. Transverse seismic waves are known as S waves. At a depth of 1000 km below the earth's surface, s waves travel at approximately 6500 m/s.

    (c) Calculate the wavelength of an s wave with an oscillation period of 1.5 s.

    The destructive power of earthquakes are often measured on the Richter scale. The Richter value for an earthquake is a number, m, equal to log(A/T) + B, where A is the amplitude of the wave in micrometers as measured by a seismometer, T is the period of the oscillation in seconds, and B is a constant that depends on the seismometer distance to the epicenter of the quake. Damage begins at m=5 and m=8.5 corresponds to the largest quakes ever measured. The Loma Prieta quake measured 7.1.

    (d) Roughly motivate the dependence of m on A, T and B. What happens if A increases? What happens if T increases? Why is there a dependence on the distance of the seismometer from the epicenter? Hint: Think about tossing a rock into a pond. What happens to the amplitude of the water ripples in a pond as the distance from the point of impact increases? What does this imply about the energy flow in waves?

  3. A 0.8 m long string is stretched and vibrates with a frequency of 20.0 Hz in its fundamental mode. The amplitude at the antinode is 0.45 cm. The string has a mass of 0.0600 kg. a) Calculate the speed of propagation of a transverse wave in the string? b) What is the tension in the string. c) What happens to the tone of the fundamental harmonic if the tension is increased?
    4.
  4. Mick Jaguar, famous rock musician, sits in the stadium during a sound check before his concert. He sits in front of the stage 20 meters from one speaker and 23 meters from another. These are the only two speakers on the stage. As part of the sound check, the frequency emitted by the speakers is swept slowly through the entire audible range from 20 to 20,000 Hz. Mick notices that the intensity of the sound he hears depends on the frequency. Should he be worried that there is something wrong with his hearing? Suppose Mick's ears have a perfectly flat frequency response. What else could cause a variation in the intensity of the sound Mick hears? At what frequencies should Mick hear a minimum intensity? At what frequencies will he hear a maximum intensity?
  5. Erving Von Humbolt, famed Professor of Pre-Columbian Artifacts has discovered a musical instrument he believes was once used by native peoples in what is now southeast Paraguay for "some serious jammin’, rockin’, and gettin’ down" during adolescent mating rituals. Unfortunately, the instrument he has discovered is broken. He comes to you for help in understanding what sounds the instrument might have made. Please help him out!

The instrument has one string. That string is tied at one end and constrained to move freely up and down a thin rod on the other end. Break up into small groups and determine the correct expression for the frequency of the nth harmonic of the string in terms of the length (L), tension (T), and the mass/length (m ) of the string. Try to convince the other groups of your answer. Below are are a few possibilities, one of which is the correct answer.

wpe1.jpg (1518 bytes)where n=1,3,5 …

wpe2.jpg (1512 bytes) where n=1,2,3, …

wpe3.jpg (1669 bytes) where n=1,2,3, …

wpe4.jpg (1525 bytes)where n = 1,3,5, …

wpe5.jpg (1495 bytes) where n=1,2,3, …