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How fast was the ball thrown?

  1. Jan 18, 2008 #1
    [SOLVED] How fast was the ball thrown?

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

    Nicole throws a ball straight up. Chad watches the ball from a window 5.0m above the point where Nicole released it. The ball passes Chad on the way up, and it has a speed of 10 m/s as it passes him on the way back down. How fast did Nicole throw the ball?

    2. Relevant equations
    Vf=Vi + at
    y=Vi(t)+.5a(t)(^2)
    Vf^2=Vi^2+2(a)(y)


    3. The attempt at a solution
    I've tried starting with the final V =0 and I've tried to solve the downward motion of the ball first, to attempt to get maybe the y value, or height of the ball at it's max. Please, where should I start? What am I missing?
     
  2. jcsd
  3. Jan 18, 2008 #2

    mgb_phys

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    A hint, since no energy is lost - the speed going back down at a point is the same as the speed going up at that same height.
    You just need v^2 = u^2 + 2as be careful of the signs!
     
  4. Jan 18, 2008 #3
    Thanks mgb_phys! I finally got the correct answer of initial velocity=14 m/s.

    for v^2 = u^2 + 2as; I let the -10 m/s of the downward velocity, u, = + 10 m/s at the same height of 5m on the upward velocity. I let v = unknown variable and the acceleration was equal to -9.8 m/s^2, and s=5m for the height. Thanks for the explanation!
     
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