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Big spring issue, need help fast

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Spring constant
    Length of compression spring
    Length of compression spring compressed all the way
    Force exerted by the spring when compressed all the way
    Weight of the projectile being projected by the spring
    Acceleration of the object while being pushed by the spring


    2. Relevant equations

    F=-kx
    F=ma

    3. The attempt at a solution

    I'm not sure, i basically have the acceleration of the ball while the springs pushing it up, but I don't know how long it takes for the spring to go from compressed to normal with the ball on it, with this i can find the speed the ball leave the spring at and that's basically what I need, and im stumped.
     
  2. jcsd
  3. Jun 10, 2009 #2

    diazona

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    umm... what's the question?
     
  4. Jun 10, 2009 #3

    LowlyPion

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    Welcome to PF.

    Your acceleration varies as the displacement decreases ... the F from the kx gets smaller, the mass doesn't.

    Consider the energy in the spring at release 1/2kx2 and consider the velocity at ejection of 1/2mv2
     
  5. Jun 10, 2009 #4
    Ok so the acceleration decreasing as the spring gets less compresses makes sense to me since its pushing with less force.

    Im still confused of how to find the velocity at ejection which is basically all I need now, from there distance and whatnot is easy enough.
     
  6. Jun 10, 2009 #5

    LowlyPion

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    Conservation of energy.

    Potential energy in spring at compression = Kinetic energy of object
     
  7. Jun 10, 2009 #6
    oh ok that makes sense to me. How do I figure out to potential energy, and from kinetic energy figure out velocity? I got kinda lost in the energy unit we did..
     
  8. Jun 10, 2009 #7

    LowlyPion

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    Well, ... I already gave you those.

    1/2kx2 = 1/2mv2
     
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