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Homework Help: Help with coupled spring and pendulum system

  1. Nov 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Given the system in the image below, I need to find the equation of motions for the coupled system. The surface where the block moves is frictionless. The red line is position where the block is at equilibrium. At equilbrium x1 and x2 = 0. After finding the equation of motion for the coupled system, I need to find the normal modes which I can do but I'm having trouble finding first part. Also the angle is small angle approximation.
    2. Relevant equations
    F= ma
    sinθ = tanθ = θ = [x2 - x1]/L

    3. The attempt at a solution
    Ok so taking the horizontal and vertical forces of each:

    Fx1 = -kx1 - Tsinθ
    Fy1 = N - mg - Tcosθ

    Mass on Pendulum:

    Fx2 = Tsinθ **************
    Fy2 = mg - Tcosθ

    *****I feel like the spring should add a force here, but I'm not sure how.
    If I proceed adding a -kx1 to Fx2 and solving for the equations of motion using the small angle approximation I get

    a1 = -k/mx1 + T/Lm(x2 - x1)

    and a2 the same which is wrong. Also I cannot get rid of T. I am not so concerned over the normal modes because I can do that after. We did not do any langrange stuff so I am not allowed to use it. Also because of the small angle, I asked if I was allowed to approximate the vertical displacement for the pendulum to be zero, but I cannot so I have to find another way.
  2. jcsd
  3. Nov 11, 2014 #2


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    Gold Member

    You have written four equations of motion, but this system only has two degrees of freedom. You should be able to eliminate some variables after you get your equations of motion correct.

    To be sure, the spring force does enter into the equation of horizontal motion for the block. How do springs generate force?
  4. Nov 11, 2014 #3
    Yeah I'm stuck here. I can't figure out how to eliminate T and I'm also unsure of how to make Fx2 correct. My problem is that I'm very limited with techniques here, as this is pretty much the beginning of an intro to classical mechanics.

    I'm pretty sure adding -kx1 to horizontal motion of the pendulum is incorrect as then the two equations that I care about are identical which is wrong. -kx2 wouldn't make much sense.
  5. Nov 12, 2014 #4


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    Gold Member

    Does the spring act on the block or on the pendulum?

    Where are your FBDs?
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