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Motion's frequency

  1. Sep 25, 2013 #1
    A electric model train travels at 31.7 cm/s around a circular track of radius 1.73 m.


    How many revolutions does it perform per second, i.e, what is the motion\'s frequency?
    ______ Rev / s

    Find the train\'s period of revolution.
    ______ s

    This is how I worked out the problem:
    v^2/r = rw^2
    .317^2/1.73 = 1.73w^2
    .317^2/1.73^2 = w^2
    w = 0.18323699421 radians per second.
    there are 2 pi radians, so it takes 2pi radians / .1832 radians/second = seconds

    34.3 seconds
    1.75 revolutions per second (60/34.3)

    I'm getting that 1.75 rev/s is wrong and I don't know why or how
     
  2. jcsd
  3. Sep 25, 2013 #2

    Dick

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    I would just use v=rw. It's a more basic formula. But it's going to give the same thing. You've got 34.3 seconds for the period ok. And 60/34.3 would be ok if you wanted revolution/minute because there are 60 seconds in a minute. But that's not what they are asking for. Can you fix it?
     
  4. Sep 25, 2013 #3
    But I thought 60/34.3 would give me the seconds? So what you're saying is that I have to convert 60/34.3 re/min to rev/sec? Isn't that what 34.3 is, it's seconds
     
  5. Sep 25, 2013 #4

    Dick

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    If a revolution takes 2 seconds, how many revolutions per second? There is really no 60 needed.
     
  6. Sep 25, 2013 #5
    Okay so if it take 2 seconds per revolution. I would take my seconds which is 34.3 and multiply it by 2?
     
  7. Sep 25, 2013 #6

    Dick

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    Noo. If it takes 2 seconds to make a revolution, then in 1 second it will make 1/2 revolution. So that's 1/2 revolution per second. Not 60/2. Just think about it. This is a different question.
     
  8. Sep 25, 2013 #7
    Okay well I'm still getting it wrong but thank you anyway
     
  9. Sep 25, 2013 #8

    Dick

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    You're welcome. But think about this. Where did the 60 in your problem even come from? I think you are just plugging into formulas without thinking about what they mean.
     
  10. Sep 25, 2013 #9
    What I understood is I get 34.3/2 and get 17.5 but that's still the wrong answer
     
  11. Sep 25, 2013 #10

    Dick

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    The 2 was just a simple example to try to get you to understand where the formulas are coming from. Here, I'll spell it out: if p is period in seconds, then 60/p is revolutions per minute. 1/p is revolutions per second. Can you understand why?
     
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