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Homework Help: Stone thrown off a cliff

  1. Nov 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A rock is thrown horizontally from the top of a 20-m high cliff and strikes the ground at an angle of 45 degrees. With what speed was it thrown?

    2. Relevant equations
    R = V2/g*sin(2theta)
    v2 = (v0*sin(theta))2 - 2gy

    3. The attempt at a solution

    I have 2 unknowns no matter what equation I try to use. I've tried to get it down to one unknown, but I keep getting the wrong answer..
  2. jcsd
  3. Nov 13, 2008 #2
    Don't think in equations, think in terms of the picture you see. You grabbed the range equation, but don't just try to plug things into it.

    The rock strikes the ground at 45 degrees. That means it's final velocity has x and y components that are related to each other how? You know x velocity is constant. You can find final y velocity using that initial y velocity is zero. If you know the relation between x and y velocity, you can then find x velocity.
  4. Nov 13, 2008 #3
    Ahhh, I think I understand now.

    v2 = 0 - 2gy
    v = (2*9.8*20)1/2
    = 19.7

    The correct answer is 20m/s. Thanks :smile:
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