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Homework Help: Physics work problem

  1. Nov 16, 2008 #1
    1. The problem statement, all variables and given/known data

    A 0.179 kg ball is thrown straight up from 1.94 m above the ground. Its initial vertical speed is 10.00 m/s. A short time later, it hits the ground. Calculate the total work done by the force of gravity during that time.

    m = .179 kg
    vi = 10m/s (upward)
    g = 9.8 m/s2
    d = -1.94 m (since the displacement is downward)

    2. Relevant equations

    W = Fd
    F = ma

    3. The attempt at a solution

    This seemed like a simple problem, I'm not sure why it's giving me so much trouble. Basically, I just need to find the force of gravity which I thought was just 9.8 * .179 but I'm clearly missing something.

    I tried... W = -1.94 * (9.8 * .179)

    Thanks
     
  2. jcsd
  3. Nov 16, 2008 #2
    [tex]x_f^2=x_i^2+2ad[/tex], [tex]W=Fd\rightarrow W=mg(x_f^2-x_i^2)/2a[/tex].
     
  4. Nov 16, 2008 #3
    That doesn't incorporate the initial velocity though does it?

    Wouldn't

    xi = 1.94 m ?
    xf = 0 m ?
     
  5. Nov 16, 2008 #4
    Ignore asleight's nonsense.

    You did almost the correct thing: you chose the displacement as negative (fine, that's just a convention), but then you should take the acceleration due to gravity as negative, too! Both are downward.

    And even without thinking about sign conventions: does gravity do positive or negative net work here?
     
  6. Nov 16, 2008 #5
    I seee, it's positive because it's in the direction of gravity.

    Thank you very much sir! :)
     
  7. Nov 16, 2008 #6
    You're welcome!
     
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