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Projectile Motion rock throw Help

  1. Jan 23, 2014 #1
    If a rock is thrown horizontally from a cliff at 25 m/s, what will its velocity be (magnitude and direction) after 3.0 seconds. The answer is 38.6 m/s, and 49.6 degrees down, but I cannot figure out how to get to it.

    Not sure which equations to use, I just started and I am learning on my own.

    I assumed that to find the velocity, you should average the horizontal and vertical velocities together. So since it has been 3 seconds, the vertical would be 29.4 m/s, because the gravitational constant is 9.8 m/s/s, and the horizontal would remain 25 m/s. Averaged, it would be 27.2 m/s. Obviously that was incorrect. As far as direction goes, I have no idea.
  2. jcsd
  3. Jan 23, 2014 #2

    Doc Al

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    Staff: Mentor

    The horizontal and vertical velocities are the components of the total velocity. You must add them like vectors, not average them. A bit of trig will allow you to find to the magnitude and direction of that vector.
  4. Jan 23, 2014 #3
    Oh okay, I'll look that up and see if I can get it. Thanks
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