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Homework Help: Kinetic Energy, Potential Energy, Work

  1. Feb 14, 2009 #1
    Hi! So this is a more general question about a question that I have already solved but I'm still confused and it seems like I'm missing the concept here.

    1. The problem statement, all variables and given/known data

    I'll try to recall the question as best I can from memory. A person is standing at a certain height h and shoots an object with some mass m at some angle z with a slingshot that acts as a spring with a k of k. The question asked for the velocity after the slingshot was released.

    2. Relevant equations

    Kf + Ugf = Ki + Ugi
    1/2mvf^2 + mgy = 1/2mvi^2 + mgh
    Ws = 1/2 kxi^2 + 1/2 kxf^2

    3. The attempt at a solution

    So basically what I did was plug in the values to find the work of the spring on the ball which I found to be 1/2kd^2. My main question is whether or not it is correct to substitute this value into the first equation above for Ki, the initial kinetic energy. This provided the correct answer and I found velocity by solving for Vf. I'm not sure if this makes any sense because I'm pretty confused myself. Thanks for any assistance.
  2. jcsd
  3. Feb 14, 2009 #2


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    Wouldn't it be easier to just use
    spring energy -> kinetic energy
    .5kx^2 = .5mv^2
  4. Feb 14, 2009 #3
    I suppose it would be--I think I've come up with my actual question now. When is it correct to substitute Work for Kinetic energy. Should I always substitute it in for Ki in the first equation and if it is vertical displacement, simply add mgh?
  5. Feb 15, 2009 #4


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    Sounds okay. I personally wouldn't bother with a W at all.
    I think in terms of energy before = energy after
    and put in whatever kinds of energy there are in each case.
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