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Homework Help: Find the Speed of the ball

  1. Sep 27, 2010 #1
    A ball is thrown horizontally from the top of a 20-m high hill. It strikes the ground at an angle of 45°. With what speed was it thrown?

    I thought that we could use h=(Vi^2sin^2alpha)/2g. However, using that formula im not able to get to the correct answer which is 20m/s. What am I doing wrong?

  2. jcsd
  3. Sep 27, 2010 #2


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    Homework Helper

    You know the formula s=s0+ut-0.5gt2 where s = displacement and u= initial velocity.

    Using this formula and considering vertical motion, the ball hits the ground at s=0. So what is the total time of flight? (If it was thrown horizontally, its initial vertical velocity is?)
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