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A frequency problem

  1. Sep 1, 2011 #1
    1. The problem statement, all variables and given/known data

    a vibrating source makes 16 pulses every 4 seconds
    find the frequency

    2. Relevant equations
    pulse is half a wave

    3. The attempt at a solution
    if we consider a pulse as half a wave then the source produces 8 waves
    then frequency=8/4=2 hertz
    but that's not the right answer
    according to the text book:
    frequency=16/4=4 Hz
    I'm completely confused can u explain the right answer?
    Thanks in advance!!
  2. jcsd
  3. Sep 1, 2011 #2


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    Homework Helper

    You are reading too much into this.

    Frequency is measure in cycles or pulses per second - unit Hertz.
    If you are given a different time unit, you convert.

    Frequency of a motor that spins at 1200 times per minute - divide by 60 = 20 hertz

    Speeding motorists are picked up at the rate of 30 per hour = (1/120) Hz

    Your example 16 pulses in 4 seconds. How do you convert to Hz?
  4. Sep 1, 2011 #3
    according to you 16/4 = 4 Hz right?
    according to you frequency is how often something repeats itself
    If so then It doesn't matter if it makes 16 pulses or 16 waves :(
  5. Sep 1, 2011 #4


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    We could talk about the frequency of the pulses, the frequency of the waves, the frequency of the traffic , the frequency of the sound, the frequency of the radio transmitter - it really doesn't matter. This problem was about the frequency of the pulses.
  6. Sep 1, 2011 #5
    Frequency simply means how often (per sec) an occurence happens over a given duration of time. For this case the occurence is the pulse. So the question is how often or how many pulses are observed every second.

    Just ignore the idea that a pulse is half wave, I'm not sure with that. A pulse is a sudden spike/burst of signal, so they are discrete and not continuous (wave). It may be given that way but not for this one.
  7. Sep 1, 2011 #6
    Yeah,I suppose I got it
    Thanks very much
  8. Sep 1, 2011 #7
    but isn't hz equivalent to cycles/second? not pulses / sec ??
  9. Sep 1, 2011 #8


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    Hertz is just "per second" its unit is even s-1

    It is how many of the things you are looking at there are per second.

    Are you looking at pulses, are you looking at waves, are you looking at revolutions of a motor, are you looking at swings of a pendulum, are you looking at traffic flow.

    Once a frequency gets below 1, we tend not to use frequency, so traffic flow is unlikely to have a Hz value associated with it.
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