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Homework Help: Free-Fall Problem

  1. Sep 9, 2012 #1
    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?

    I'm not asking for answer, I know the Physics Forum rules, but I am totally unsure where to start. I guess I have to use the equation d = Vot + at²/2 ?

    Could you please give me a hand and help me figure it out, please. Hints is all I need. Thanks.
  2. jcsd
  3. Sep 9, 2012 #2
    lemme try

    you are given initial velocity =0
    acceleration is 10/9.8m/s2
    let total height be h
    and time taken to cover full length be t+1 sec

    now find distance travelled is t seconds and then find it for t+1 sec

    then u get 2 equations involving time and height

    solve for answers...
  4. Sep 9, 2012 #3
    Why t+1, though?
  5. Sep 9, 2012 #4
    now distance traveled in last sec is known

    so u can take total time as t and 1 sec before time time as t-1


    u can take total time as t+1 and 1 sec before time as t
  6. Sep 10, 2012 #5
    Or just solve for V_0 with d = 1/3 h and t = 1 s. Then go back and solve for the time it takes to attain that velocity in free fall.
  7. Sep 10, 2012 #6


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    Homework Helper

    h = 1/2*g*t^2 --------(1)
    h - h/3 = 1/2*g*(t - 1)^2 -------(2)
    Divide 1 by 2 and solve for t. Hence find h.
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