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Waves Lesson

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Wave Basics

What are waves?

All waves are the movement of energy through a medium (Light, Sound). They transmit energy, not water mass, so the water is not actually moving. They often occur at a boundary between and within fluids with different densities:

Wave Anatomy (true for all waves)

Crest . . . . . . . . . . . . . . . . . . . .
Trough. . . . . . . . . . . . . . . . . . .
Wave Height . . . . . . . . . . . . . .
Wavelength . . . . . . . . . . . . . . .
Amplitude. . . . . . . . . . . . . . . . .
Baseline. . . . . . . . . . . . . . . . . .
Period . . . . . . . . . . . . . . . . . . .
Frequency . . . . . . . . . . . . . . . .
Steepness . . . . . . . . . . . . . . .
Velocity = wavelength/T . . . . . .
Highest part of a wave
Lowest part of a wave
Vertical distance from crest to trough
Distance from Crest to Crest
Distance from the Crest or Trough to the baseline
aka “still water” line
Time required for a wave crest at point A to reach point B
Number of waves which pass point A per second
Ratio of height of wave to its wavelength (Height/wavelength)
Example:
Given T=10 sec; wavelength = 100m
V=wavelength/T = 100m/10 sec = 10m/sec

Not all waves travel at the same speed. The smaller the wavelength the slower it moves and the larger the wave the faster it moves.

Classifying Waves

Ways to classify waves:

  1. Disturbing force
  2. This is the force that creates the wave.
    Wind waves - formed by wind moving over the water.
    Seices - Change in atmospheric pressure over the water, storm surges, etc.
    Tsunami - disturbance of the seafloor
    Tides - gravitational attraction of the Earth, moon and sun.

  3. Free waves vs. forced waves
  4. Free waves move without any further influence from the force that created them. Wind waves are an example of this. Wind blowing over the water forms the waves, but the waveform will continue on for thousands of miles after the wind has stopped.
    With forced waves, movement is dependant upon the force that creates them. Tides are an example; were there no gravity there would be no tides.

  5. Restoring force
  6. The dominant force trying to return the water surface to flatness after a wave has formed. If successful, energy would be transferred as heat not a wave.

  7. Wavelength
  8. Small waves (wavelengths < 1.73 cm) When wind blows over the ocean, energy is transfered to the water. Surface tension between the water molecules and makes the area move as one mass. These small waves are called capillary waves, and are the first waves to form when the wind blows.

    Larger waves (wavelengths > 1.73 cm) form as the wind continues to blow, which causes the wave height to grows. Gravity begins to pull the crest down. The wave continues to move forward, so crest becomes a trough. This causes circular orbits of individual water molecules. Once these waves form they can travel for thousands of miles.

  9. Other types of waves
  10. Splash wave - form from coastal landslides, calving icebergs; basically from an object falling into the ocean to make a "splash".

    Wake - these waves from as a object passes through the surface, such as a duck, a ship, etc. The water is parted, forming the wake.

    Internal waves - These waves are associated with the pycnocline. They are larger than surface waves and are caused by tides, turbidity currents, winds, ships, and oil slicks. Internal waves are a possible hazard for submarines as, unlike surface waves, they cannot be easily seen.

    Atmospheric Waves - Atmospheric waves are somewhat like internal waves, but in the atmosphere. They form due to disturbances in the atmosphere from things like storm fronts and pressure gradients.
    Video of an undular bore waves over Iowa, Oct. 3, 2007


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