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Homework Help: Finding a spring constant of a car

  1. Sep 21, 2012 #1
    1. The problem statement, all variables and given/known data
    A car has a total mass of 2000kg, and a ground clearance of 40cm. But some fat dude got in and he made the ground clearance 30cm. What is the spring constant for this bad boy of a car? (The dude was so fat he made the shocks act like springs if you can believe it.) Also, it's a fact that the natural frequency of said springs are .6667 HZ.

    2. Relevant equations

    F = -kx

    3. The attempt at a solution

    So I tried being like: (2000kg)*(9.8m/s^2) = -kx
    and then I was like: (2000kg + Xkg) * (9.8m/s^2) = -k(x-.1m)

    I tried solving for k as hard as I could cause I really want it, but there are too many variables and I can't figure out what to do. My physics teacher says he wants me to fail and I'd like to show him that I'm not a total turd. Thank you.
  2. jcsd
  3. Sep 21, 2012 #2

    Simon Bridge

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    The car weight compresses the springs to 40cm, but it is the car+dude that compresses the springs to 30cm. So the extra weight of the dude compresses the springs an extra how much?

    Your trouble is that you don't know the weight of the dude or the spring constant ... that's two unknowns: so you need two equations.

    You also need the equation for the natural frequency of a mass on a spring.
  4. Sep 22, 2012 #3
    Ok Physics bros... I think I had a stroke of genius, thanks to Simon Bridge, but I'm wondering of the mathematical/physical legality of what I've done.


    (2000kg)*(9.8m/s^2) = -kx = (2000kg)*(g) = -kx

    (2000kg + Xkg) * (9.8m/s^2) = -k(x-.1m)= (2000kg +Xkg) * (g)

    Is it legal to do this...

    (-k(x-.1m)= (2000kg +Xkg) * (g)) - (-kx = (2000kg)*(g)) = (X*g = -.1k)

    Then say that k = -(X*g)/.1

    Cause if that's k, I can plug it into other formula and I'm good to go.
  5. Sep 22, 2012 #4

    Simon Bridge

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    Are you confusing the clearance with the compression?

    Let the uncompressed clearance be x0 - then the car weight Mg compresses the springs by x, giving a clearance of 40cm ... eg.

    40cm = x0-x: kx=Mg ... thus: 40cm = x0-Mg/k

    similarly for Car + dude: 30cm = x0-(M+m)g/k

    But you don't know x0, m, or k ... so that's two equations and three unknowns.

    You are still missing the equation for the natural frequency of the spring.
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