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Homework Help: Projectile Motion - Horizontal Component

  1. Apr 25, 2010 #1
    A rock is thrown off a 100.0m cliff upward at an angle of 20o to the horizontal. It has an initial velocity of 15ms-1 and strikes the rocks below.

    Find how far away the rock is from the base of the cliff.

    I calculated total time of flight (projectile parabola + 100m vertical descent) which equalled around 4.3-5.0 (verified by textbook.)

    The total time of flight for the parabolic section was about 1.046s

    Horizontal component = 15cos20 = 14.095ms-1

    SH = ut = 14.095 x 1.046 = 14.74m.

    However, the answer is 70m and you can only acquire that by 14.095 x 5 which doesn't make sense because I thought after being projected at an angle it would make a completely vertical descent thus having no effect whatsoever on the horizontal component.
  2. jcsd
  3. Apr 25, 2010 #2


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    Exactly what do you mean by "parabolic section"? Is the entire trajectory of the rock a parabola or only a piece of it?
  4. Apr 25, 2010 #3
    I got t equals 4.022 taking g as 9.81. from that s=ut+1/2at^2.... where u = 15cos(20) and a = 9.81... comes out to 76.43m. ignoring drag.
  5. Apr 25, 2010 #4
    sorry didnt see the answer you provided the book is right but i assume that you take gravity as 10m/s^2
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