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Homework Help: Projectile motion rock off a cliff

  1. Nov 23, 2013 #1
    1. The problem statement, all variables and given/known data
    A rock is thrown down off a cliff at 18 m/s at an angle of 17 below the horizontal. If the rock hits a cliff that is 16m away, how far down did the rock fall.

    2. Relevant equations
    Vf= Vi + at
    change in Y= Vit + .5at^2

    3. The attempt at a solution
    I tried solving for time and then plugging that back into the equations and I kept getting -3.87. Please help!
  2. jcsd
  3. Nov 23, 2013 #2


    User Avatar
    Gold Member

    You can eliminate time by writing an equation for the x displacement of the rock and subbing this into your equation for it's y displacement.
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