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Homework Help: Physics problem, involving Hookes Law?

  1. Apr 2, 2013 #1
    1. The problem statement, all variables and given/known data
    A 0.500 kg mass is resting to a horizontal spring constant of 45 N/m. Your lab partner pulls the spring back and releases it when you are not looking. When the spring reaches its equilibrium point (x=0) the velocity of the mass is 3.4 m/s.

    Find how far your partner compressed the spring.

    2. Relevant equations
    Ep = 1/2 k x ^2
    k = spring constant
    Ek = 1/2 m v ^2

    3. The attempt at a solution
    Not really sure how to do this, since there is more than one unknown.

    Ep1 = Ep2 + Ek2

    They never stated the spring stops at x=0, and I'm willing to bet it doesn't. However, when I assume that Ep1 = Ek2 and assume it stops, I get 0.30 m (rounded) when the answer should be 0.19m

    Am I doing this right if I just set Ep1 to Ek2? It doesn't make sense to do it like this, because it should be

    Ep1 = Ep2 + Ek2

    But then we don't have enough info to solve the equation
  2. jcsd
  3. Apr 2, 2013 #2


    User Avatar
    Homework Helper

    What is the elastic potential energy at the equilibrium point (x=0)?
    Check the first sentence of your post. Something is missing.

  4. Apr 2, 2013 #3


    Staff: Mentor

    what you're really saying is that at x=0 the total energy of the system is just the KE because the spring is at its equilibrium position and hence there's zero PE and initially there's no KE and since energy is conserved.

    PE1 + KE1 = PE2 + KE2 where KE1=0 and PE2=0 so PE1 = KE2 seems right.

    I calculated the compression to be 0.358 m so I'm thinking maybe the book answer is wrong or there's some assumed friction component.
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