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Homework Help: Frequency of waves

  1. Aug 3, 2006 #1
    There are two parts to this question. I got the first part and thought the second would be relatively easy to answer, but I keep getting it wrong:

    Ocean waves are traveling to the east at 3.2 m/s with a distance of 22m between crests.

    a) With what frequency do the waves hit the front of the boat when the boat is at anchor? ** 3.2/22=.1455 Hz, which I got right.

    b) With what frequency do the waves hit the front of a boat when the boat is moving westward at 1.4m/s?

    Now, b/c they are moving towards each other, I substrated 1.4 from 3.2 and I got 1.8. I then got 1.8/22=0.0818, which is incorrect.

    What am I doing wrong with regard to the second part?
  2. jcsd
  3. Aug 3, 2006 #2
    If you are walking towards a wave, will the wave hit you more frequently or less frequently? Will it seem like the wave is moving faster or slower?

    Similarly, will a boat moving towards the wave be hit with a higher or lower frequency than it would were it at rest?

    Ask yourself these questions, and you will see if you did something incorrectly.
  4. Aug 3, 2006 #3
    Ok. It would be hit at a greater frequency, so I should add instead of substracting?

    That confuses me... because I thought that if two waves were moving toward each other it was destructive (or something like that) and so you would subtract from one another. At least, that's how I understood what my prof was saying.
  5. Aug 3, 2006 #4
    But is the boat a wave?

    When two waves collide, they can cancel each other out if their characteristics meet certain requirements. You understood your professor correctly.

    (Disclaimer: It turns out that the boat (and all matter) has a wave component given by one of de Broglie's equations. But you don't need to worry about that, as it is a negligible component.)
  6. Aug 3, 2006 #5
    I just assumed I could consider the boat a wave. Not sure why.

    So -- 1.4 + 3.2 = 4.6/22 = .2091

    Is that correct then? I only have one more chance to answer this question correctly, which I why I'd just like to make sure, if that's possible.

    *EDIT* Actually, that's wrong. That was an answer that I tried earlier...
    Last edited: Aug 3, 2006
  7. Aug 3, 2006 #6
    The logic is sound, and the math appears correct. The question is - do you think it is the right answer?
  8. Aug 3, 2006 #7
    Yeah, it's not right. I tried that answer earlier. Apparently it's too small.
  9. Aug 4, 2006 #8
    my approach is nothing different than what has been done before, but assume the boat is at a point B 22 m eastwards of point A where the first wave is approaching.

    When the wave collides the boat they both have travelled a distance total of 22 m, thus:

    [tex]3.2\cdot t + 1.4 \cdot t = 4.6\cdot t = 22 \Rightarrow t = 4.8 s[/tex]

    Then the next wave is again at a distance 22 m and collides after 4.8 seconds.

    --> thus the frequency = 1/4.8 = 0.208 Hz
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