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Waves Frequency

  1. Jan 17, 2016 #1
    1. The problem statement, all variables and given/known data
    A cord vibrates with a frequency of 3.0 Hz when a mass of 0.60kg is hung up from it. What is its frequency if only 0.38kg hangs from it?

    2. Relevant equations

    f= 1/2pi sqrt k/m
    3. The attempt at a solution
    3.0= 1/2pi sqrt ( k/0.6)
    k=213.18 N/m

    f= 1/2pi sqrt (213.18/ 0.38)
    f= 37.2 Hz
     
  2. jcsd
  3. Jan 17, 2016 #2
    The ratio between the masses is 60:38, so the ratio of frequencies is the square root of that.

    But what is this about: an oscillating mass on an elastic string or a vibrating cord under tension?
     
  4. Jan 17, 2016 #3
    @PietKuip it just says an elastic cord vibrates when a mass is hung from it. Also, what is wrong in my calculations is it that you are supposed to use grams for the mass?
     
  5. Jan 17, 2016 #4
    Actually it can't be because I used kg that is what you are supposed to use.
     
  6. Jan 17, 2016 #5
    Units don't matter, just use the ratio.

    It is quite a difference if the mass is oscillating up and down on a string or whether the string is in a transverse oscillation without the mass moving. Different kinds of motion, different formulas apply.
     
  7. Jan 17, 2016 #6
    @PietKuip I understand that the ratio method will work. But I am a bit concerned why what I did above did not work as my textbook specifically said to use that formula.
     
  8. Jan 17, 2016 #7
    Weird textbook. How can one learn any physics that way? The wording suggests a different mode of motion than for the case where this formula applies.
     
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