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Physics Problem with Spring Constant and Kinetic Friction

  1. Jan 3, 2015 #1
    1. The problem statement, all variables and given/known data
    A person attaches a spring to an cubic object that weighs 36kg and pulls this object along a table made of material X horizontally with a steady speed of 1.3m/s. The spring stretches a distance of 3.4cm. Find the ratio of the spring constant to the kinetic friction coefficient.

    2. Relevant equations
    I suppose FkkN
    Fspring=-kx
    F=ma
    Not sure if any other ones are needed, but I couldn't find a way to solve this and get a reasonable answer.
     
  2. jcsd
  3. Jan 3, 2015 #2

    Bystander

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    What does this information tell you?
     
  4. Jan 4, 2015 #3
    I'm not sure, but I tried to apply it to the F=MA formula... Am I on track?
     
  5. Jan 4, 2015 #4

    Bystander

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    Okay, we'll work it through that way: "Steady speed" means what in terms of "A?"
     
  6. Jan 4, 2015 #5
    Acceleration=0 at steady speed, but the trouble I'm having is converting everything to Newtons, since spring constant is Newtons/meter, and kinetic friction coefficient is Force of Kinetic Friction/Normal Force. The problem didn't give any units related to those calculations, maybe there is a way to convert it but I have yet figured it out/learned it.
     
  7. Jan 4, 2015 #6

    Bystander

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    This looks perfectly useful. What's the normal force?
    Nothing wrong with this.
    You aren't required to generate a numerical answer for every problem on the planet. Sometimes it's just a matter of coming up with a symbolic expression.
     
  8. Jan 4, 2015 #7

    haruspex

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    All good so far. Now, what is the relationship between F, Fk and Fspring?
     
  9. Jan 5, 2015 #8
    F=Fk+Fspring right?
    If that is so then:
    F=MA, A=0, and F=0
    So 0=Fk+Fspring
    0=-kx+μkN
    kx=μkN
    k/μk=N/x, since we are trying to find the ratio between the spring constant and the kinetic friction coefficient
    Since N, normal force, is 36*9.8N right now, and x, distance, is 3.4cm or 0.034m right now, then it is 36*9.8/0.034=10,376.47, which is the ratio of spring constant to kinetic friction, is that correct?
     
  10. Jan 5, 2015 #9

    haruspex

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    Yes, that all looks right. But you should include the units in the answer.
     
  11. Jan 5, 2015 #10
    Thanks!!!
     
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