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Point in water wave with greatest upwards speed?

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  1. Aug 8, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-8-8_23-44-4.png

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    3. The attempt at a solution
    I chose D because I thought that since it is at the lowest point on the wave, it will move upwards with the greatest speed. But I'm wrong, since the answer is C. Is the answer C because of some time of acceleration from point D to C? If so, then why is the answer not B instead?

    Or the water waves here can be compared to the swinging pendulum, where velocity is greatest at its equilibrium position. I have no trouble understanding the pendulum case but I can't picture the velocity of the water waves at their different positions.
     
  2. jcsd
  3. Aug 8, 2015 #2

    Orodruin

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    The situation is exactly analogous to the pendulum. At B the water surface is rising the fastest (assuming that the entire wave moves with the given speed). If it helps, you can try drawing the position of the wave just a small time later and see how the surface has moved.
     
  4. Aug 8, 2015 #3
    Errm, so you mean that the speed of water wave is fastest at B, not C?
     
  5. Aug 8, 2015 #4

    Orodruin

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    No, sorry, that was a typo.
     
  6. Aug 8, 2015 #5

    rcgldr

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    If you want to use a pendulum analogy, turn the pendulum sideways. The pendulum comes to a stop at B and D, and is moving fastest at A and C. The question asks for when the water is moving fastest upwards, so the answer is C. The image shows the wave to be shaped like a sin wave.
     
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