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Homework Help: Wave propagation with water moving

  1. Aug 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Suppose a person is fishing and is bobbing the fishing pole in constant intervals, there are 2 fish in the water located in the same distance L from the fishing pole (the source of the wave).
    Suppose also the water is moving from A to B. Which fish will feel the wave first? Snapshot.jpg

    2. Relevant equations & The attempt at a solution
    Firstly I hope the problem statement is clear as it was translated. Do you believe the answer is uniquely determined? To me it seems to depend on the velocity of the water and the the period Of the wave.
    If only one wave was created and the water was moving with the fish with it, both fish would feel the wave in the same time.
    But if the water was moving fast, with the appropriate period of the wave, you might create a situation where fish A would get hit by the second wave before the first wave. (Perhaps by hitting the fish with the hook of the fishing pole)

    The answer given is both fish would feel the wave in the same time by considering the reference frame of the water. But does this answer really considers the case I gave?
  2. jcsd
  3. Aug 24, 2015 #2


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    You can assume that the water is not moving faster than the velocity of the waves.

    Also, I think the fish stay in place.
  4. Aug 24, 2015 #3
    In addition to what mfb said, don't think of every exception to the answer. Think about the most likely situation (which would not include the water moving faster than the velocity of the waves). I used to think about these kind of problems too hard, and my science teacher got really mad.
  5. Aug 25, 2015 #4


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    mfb, I agree with your first advice above, but I'm not so sure about the second. Anybody who goes angling in streams will be aware that fish do tend to swim against the current so as to stay more-or-less in one spot for a while, but that does not necessarily apply to the author of the puzzle. I have a nasty feeling it may be intended as a trick question.
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