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Homework Help: 2 Dimensional Motion and Vectors Help

  1. Nov 11, 2007 #1
    1. The problem statement, all variables and given/known data

    A person standing at the edge of a seaside cliff kicks a stone over the edge with a speed of 18 m/s. The cliff is 52 m above the water's surface. How long does it take for the stone to fall to the water? With what speed does it strike the water?

    2. Relevant equations

    3. The attempt at a solution

    So far, I was able to figure out the time:

    dy= 1/2gt^2 where g=9.81 m/s^2
    52m= 1/2*9.81*t^2
    t= 3.26 seconds

    I thought it would be correct to use Vy=gt to find the speed at which the stone strikes the water, therefore being (9.81 m/s^2)(3.26 s)= 31.98 m/s. But when i checked the answer key and talked to the teacher, she said that the answer was 36.889 m/s, and I was wrong.

    So can someone tell me what I'm doing wrong and how to find the speed at which the stone strikes the water?
  2. jcsd
  3. Nov 11, 2007 #2
    You have only calculated the vertical component of the speed.

    You need to do a vector addition of this with the horizontal component.
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