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Homework Help: Classical Mechanics Problem

  1. Apr 23, 2009 #1
    1. The problem statement, all variables and given/known data

    Give the equations of motion of the following system:


    2. Relevant equations

    So, i assume the following cases (the diagram i so deficient).

    1) Black Point (x1) is fixed
    2) There's a force applied at x1 (black dot)
    3) The position of the black dot is a function of time f(t).

    3. The attempt at a solution

    For 1) I think the equation of motion is:

    http://www.texify.com/img/%5CLARGE%5C%21m%5Cddot%7Bx%7D%2Bkx%2BG%5Cdot%7Bx%7D%3D0.gif [Broken]

    It's a dampeda harmonic oscillator, Right?

    2) If a force f(t) is applied to the system, then we have :

    http://www.texify.com/img/%5CLARGE%5C%21m%5Cddot%7Bx%7D%2Bkx%2BG%5Cdot%7Bx%7D%3DF%28t%29.gif [Broken]

    like the Damped and driven harmonic oscillator, am i right?

    3) If by example, the full system (the damper too) is moving to the left (i.e. negative X).

    For this, i tried the following:

    The position of the mass is X=f(t)+L+lo , where L is the natural lenght of the spring, lo is the elongation or strecht of the spring. Then, taking the time derivatives of X, and substituing in the damped harmonic oscillator equation (Case 1), i get to:

    http://www.texify.com/img/%5CLARGE%5C%21m%5Cddot%7Bf%7D%2Bk%28f%28t%29%2BL%2Bl_0%29%2BG%5Cdot%7Bf%7D%3D0.gif [Broken]

    Please, tell me if i am right in the whole problem. Thanks
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 23, 2009 #2
    I've not done this for a while.. but.. resolve the forces around the mass M
    I get something like (cant get latex thing workings so decript!)
    m(x''2) - G(x'2) + k(x1 - x2) = 0

    then you know x1 is a function of time f(t)
    so this can be substituted in for x1 i believe
    then rearrange so f(t) is subject.
    I may be slightly wrong.. but I had a go :)
  4. Apr 23, 2009 #3
    P.S. force from spring = k( x1- x2)
    which is the bit that may have confused you
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