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Homework Help: Oscillating Mass between Two Springs

  1. Feb 5, 2010 #1
    1. The problem statement, all variables and given/known data

    A block on a frictionless table is connected to two springs having spring constants k1 and k2. Show that the block's oscillation frequency is given by f = (f12 + f22).5 where f1 and f2 are the frequencies at which it would oscillate if attached to spring 1 or spring 2 alone.

    2. Relevant equations

    f = 1/T
    T = 2pi(m/k).5

    3. The attempt at a solution

    I have substituted the period into the frequency and I am stuck. It looks like they found f1 and f2 alone and put them into the Pythagorean theorem, but I do not know how to relate them.
  2. jcsd
  3. Feb 5, 2010 #2
    I do not see any relation to Pythagorean theorem. Notice that if you were to latch two springs onto a mass, you could model the same motion with one spring with a constant equal to the sum of the two springs. Knowing this, calculate the frequency of all three configurations(spring 1, spring 2, and spring 1 & 2 combined). Next, substitute your answer for spring 1 and spring 2 into that equation and see if you get the same answer you derived for spring 1 & 2 combined.
  4. Feb 5, 2010 #3
    When I find the frequencies of the springs, I do not understand how to relate them.
  5. Feb 5, 2010 #4
    I told you how.

    Calculate frequency 1 with k = k1, frequency 2 with k = k2, and f with k = k1 + k2.

    [tex]f_1 = \frac{1}{2\pi} \sqrt{\frac{k_1}{m}}[/tex]
    [tex]f_2 = \frac{1}{2\pi} \sqrt{\frac{k_2}{m}}[/tex]
    [tex]f = \frac{1}{2\pi} \sqrt{\frac{k_1 + k_2}{m}}[/tex] because [tex]k = k_1 + k_2[/tex] and nothing else changes.

    Next, plug [tex]f_1[/tex] and [tex]f_2[/tex] into [tex]f=\sqrt{f_1^2+f_2^2}[/tex] to see if you get the same expression as above for [tex]f[/tex].
    (you do)
    Last edited: Feb 5, 2010
  6. Feb 5, 2010 #5
    Oh, I was going a different route using a different equation making it more complicated. Thank you for your help.
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