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Frequency, and finding wave speed. Professor mistake?

  1. Jul 10, 2010 #1
    I am studying for an upcoming test, and it is known that there are hiccups in our study guide. I am stuck on two problems.

    1. The problem statement, all variables and given/known data

    1. What is the frequency, in Hz, of a sound wave (v=340 m/s) with a wavelength of 10 m?

    2. A series of ocean waves, each 8 m from crest to crest and moving past an observer at a rate of 2 waves per second, has what speed?

    2. Relevant equations

    1. Frequency= 1 / Period

    2. V = Lambda / Period

    3. The attempt at a solution

    1. I get the answer .029, but the study guide says 0.0029 Hz?

    2. I get the answer 4, but study guide says 16?

    Any help is appreciated!
  2. jcsd
  3. Jul 10, 2010 #2


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    Can you show your actual calculation? I don't get either of those answers.

    One wave (crest to crest) is 8 m, so two waves would be ____m?
  4. Jul 12, 2010 #3
    Sure thing

    1. Sorry...slight mix up in answers, the sheet says .029 but I am getting .0029. 1 / 340 = .0029. Am I even doing the right formula for this?

    2. OK, so it is 16. Why am I using V=Lambda x Period and not V=Lambda / Period?
    Last edited: Jul 12, 2010
  5. Jul 12, 2010 #4
    Both of these answers are wrong unless you read it wrong.You need to combine both formulas to get the answer.

    No. V=Lambda / Period and V=lambda*frequency.
  6. Jul 12, 2010 #5
    For problem one, I notified the professor and he said the guide is incorrect...the answer is 34. Now it all makes sense for that one!

    For problem two, whoops >.<

    I take it I only use V=Lambda*Period and f=1/Period if I am only given the period and I'm trying to discover Velocity or Frequency (respectfully).
  7. Jul 12, 2010 #6
    1) Start by getting an equation in the right form for what you are trying to find. You are looking to find Frequency from Wavelength (i.e. Lambda) and Velocity, so your expression must contain all these quantities. If you do that you should get the expected answer.

    2)Using V = Lambda / Period, the Period is not 2;

    "2 waves per second" means Frequency = 2 Hz. Period = 1 / Frequency, so Period= 0.5s
  8. Jul 12, 2010 #7
    No V is never Lambda*Period. Look at the units length*time is not velocity.In this case you get the correct number but not the correct answer.Always look at your units and numbers and think if they make any sense. Also if you have equations containing the same terms you can always combine them but be careful with the math.
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