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Homework Help: Spring extension

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data

    I'm writing a paper on one of my lab experiments and I'm not sure my physic concept is right? The question asks if the mass of a cart affects the extension of the spring. A spring is attached to a cart which is attached to sting through a pulley and connected to a hanging object of mass W (a picture is attached). From my results, the spring stretched slightly more with the cart than with out the cart attached to the spring.

    2. Relevant equations

    I know that at equilibrium all forces equal 0. Σ Fx = Σ Fy = 0
    Fs= kx

    (with cart)
    Σ Fx = Fs –Th
    Σ Fy = Tv + Fn - Mgw -Mgc

    (without cart)
    Σ Fx = Σ Fx = Fs –Th
    Σ Fy = Tv - Mgw

    3. The attempt at a solution

    If k is constant and Fs= kx and F=mg then mg=kx. Then if the cart was attached, mass will increase, therefore x (the spring extension) will increase, which explains why the spring stretched more when the cart was attached.

    Is this somewhat right, because he said i have the wrong concept? Can anyone help me? Am I missing something?

    Attached Files:

  2. jcsd
  3. Oct 13, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    This is a bit confused since it's not clear what object you are analyzing. You don't just add up all the forces that appear in a problem, you must pick an object (the cart or the end of the spring) and analyze the forces acting on it.
    Here "m" is the hanging mass, which doesn't change when the cart is attached. So adding the cart would not affect the stretch of the spring.
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