Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple Harmonic Motion- From Uniform Circular Motion

  1. Feb 10, 2009 #1
    1. The problem statement, all variables and given/known data

    I don't have a homework question exactly, but I need help with an equation please.

    Angular frequency: W= 2 pi/T = 2(pi)(f) f= frequency

    And- W = square root of (k/m) k = spring constant m= mass

    So, wouldn't T = 2 pi / square root of (k/m) ??

    My instructor has given us the following equations in class a few times. I cannot figure out what the heck is going on!

    W = (2 pi) x the square root of(k/m)
    T = (2 pi) x the square root of (m/k) -------> m/k this time

    Any help is appreciated. Im so confused right now.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 11, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is simply because of the fact that:

  4. Feb 11, 2009 #3
    Ok- but in the book it gives: W = sq. root of (k/m)

    Instructor gives: W = 2pi x sq. root (k/m)

    Whats with the 2 pi??

  5. Feb 11, 2009 #4
    What's the difference between measuring angular frequency in Hz and measuring frequency in radians per second? That might give you an idea where the conversion comes from.
  6. Feb 11, 2009 #5
    Ok...I'll sit down with that thought. So, either one works right? They are the same thing??
  7. Feb 11, 2009 #6
    Ah I see what you're getting at now. There's a mistake in the equation for frequency:

    [tex]\omega_{n} = \sqrt{\frac {k}{m}}[/tex] (1)

    where frequency is in radians per second.

    But what if you want to express the frequency in Hertz? Well, we know that 1 Hz is equal to one cycle per second. In the case of circular motion, one cycle is equal to [tex]2\pi[/tex] radians.

    So to convert from radians per second to Hertz, one must divide by [tex]2\pi[/tex]. Hence:

    [tex]\omega = \frac {1}{2\pi} \sqrt{\frac{k}{m}}[/tex] (2)

    where frequency is now in Hertz.

    Now let's express this in terms of the period of one cycle, T. Bear in mind that if you were simply to reciprocate the expression for frequency when expressed in radians per second (equation 1), you would be stating the length of time of rotation for one radian alone. Hence you have to multiply the expression by [tex]2\pi[/tex] now to obtain the period for a single cycle. This is now the same equation as you would obtain by reciprocating equation 2.

    Hope this helps.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook