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Homework Help: Freefall Problem

  1. Aug 20, 2005 #1
    hi. i've been trying to do this problem for about 5 hrs...and i still can't get it! :bugeye: If anyone of you could please point me in the right direction, i'd really appreciate it.

    The Problem is: A rock dropped from a cliff falls one-third of its total distance to the ground in the last second of its fall. How high is the cliff?
    Answer Given = 145.7 m

    I've tried drawing a diagram, substituting, but im still missing some concept! i can't get the answer given.

    I know the 4 kinematic equations commonly used in class are
    1. X = Xo + Vavg*t
    2. X = Xo + Vo*t + (1/2)*a*t^2
    3. V^2 = Vo^2 + 2*a*deltaX
    4. V = Vo + a*t

    i know, that since its falling off a cliff, the acceleration = g. and i have a picture but i don't know how to draw it on the computer. i've been trying to find the final velocity of the rock as it travels 2/3 down the cliff, because that final velocity would be the initial velocity of the rock as it falls the last 1/3 in a time of t= 1 second. But i can't figure out the time it takes the rock to travel the first 2/3 down the cliff. so in conclusion, i don't know what im doing. several pages of random tries and still nothing.... thx in advance.

    - frustrated high school student :confused:
  2. jcsd
  3. Aug 20, 2005 #2


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    Homework Helper

    Using your eqn 2),
    dist fallen in t secs is x
    dist fallen in (t-1) secs is (2/3)x
  4. Aug 20, 2005 #3


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    When you substitute Xo = Vo = 0 and a = g, then you have 2 Eq'ns in 2 unknowns (X and t):

    X = (1/2)*a*t^2
    2X/3 = (1/2)*a*(t-1)^2
  5. Aug 20, 2005 #4

    thank you very much for your help Fermat and EnumaElish. I c how to work the problem now. till we meet again, bye. :rofl:
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