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Homework Help: Calculating spring constant

  1. Mar 30, 2010 #1
    1. The problem statement, all variables and given/known data

    A mass 1 kg mass is hung from a spring.
    Calculate the spring constant from the given data. Velocity initial= 3.0 m/s Velocity final= 1.5 m/s. The initial height traveled is 0.1 meters and the final is 0.25 meters. These height values will be negative since i picked my starting point at the origin so they have moved below the X making them negative.

    2. Relevant equations
    I know i am supposed to use the conservation of energy equation with the potential elastic factored into the total PE. the equation would be (1/2*m*v^2)+(mgh)+1/2*k*x^2)=(1/2*m*v^2)+(mgh)+1/2*k*x^2) where the left is the initial and the right is the final. However i keep getting the wrong answer. I think I am doing a simple algebraic error in my calculations. any guidance is appreciated

    3. The attempt at a solution
  2. jcsd
  3. Mar 30, 2010 #2

    jack action

    User Avatar
    Science Advisor
    Gold Member

    I think you have to determine the initial displacement of the spring under the static load:

    Xo = mg / K

    Then final displacement is:

    Xf = mg / K - [tex]\Delta[/tex]X

    Where [tex]\Delta[/tex]X = 0.25m - 0.1m = 0.15m

    Put that in your equation and do the transformations and it should give you the correct answer.
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