Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    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


    User Avatar
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook