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Sum of the forces problem

  1. Apr 30, 2008 #1
    1. The problem statement, all variables and given/known

    1. The problem statement, all variables and given/known data
    There is a spring with two crates, (5kg +3 kg) as shown in this picture. (1)
    The spring constant is 1000 n/m.

    I need to find
    a) How much the spring is compressed from its initial position (at rest)
    b) The frequency of the oscillation
    c) Max amplitude after contact

    And then there is a different spring, oscillating horizontally with 2 boxes on top of each other, m1 on m2. The maximum force of friction is define as f.

    I need to find the a) max horizontal acceleration for the m1 not to slip on m2 and I need to find the b) max amplitude for simple harmonic motion without m1 slipping. I'm not given any numbers whatsoever.
    3. The attempt at a solution

    For a) I would think I would do (8)(9.8) and then do that against the force of the constant
    b) I think I would use w (frequency?) = (k/m)^1/2.
    c) I'm not too sure on this

    I don't know what to do afterwards (or at all) for the second spring

    Attached Files:

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    Last edited: Apr 30, 2008
  2. jcsd
  3. May 1, 2008 #2
    Any advice to kick me off?
  4. May 1, 2008 #3


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    Homework Helper

    Hi simplemail1,

    For part a of the first problem, you can definitely use the sum of the forces; however, the acceleration is not zero so you would need to be able to tell what the acceleration is and set the sum of the forces equal to [itex]ma[/itex]. As an alternative, I would try using conservation of energy to find the answer.
  5. May 1, 2008 #4

    Exactly, all of these questions can be solved from the Newton's equation F=ma.
    a) In this case, the equilibrium position is asked, so a=0 and sum of forces=0. In the sum of forces, you have the gravitation force mg and the reaction of the spring k(l-l0). In those kind of problems always be aware of the difference between the position at rest and the natural length of the spring l0 that are different due to the gravitation
    b) F=ma allows you to obtain the differential equation of movement, mx''+kx=0 so you can have the pulsation
    c) solving the differential equation allows you to have x(t), x'(t) and x''(t), which all will be sinusoidal functions... It is easy to have their maximum

    For the second spring, always the same problem, but the forces will be different: no gravitation, but friction force.
  6. May 1, 2008 #5


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    Homework Helper

    Hi jonpoux,

    The wording of the question is not too clear, but I don't think they are asking for the equilibrium position in this problem. They are setting up an oscillation so I think they want the maximum distance the spring is compressed from it's initial position during the oscillation.


    It doesn't look like you typed in the complete statement of the problem. It might be best if you type in everything the problem said; little details can make a big difference.
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