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Rock Drop Free Fall Problem

  1. Dec 14, 2008 #1
    1. To determine how high a cliff is, a llama farmer drops a rock, and then 0.800 s later, throws another rock straight down at a velocity of −10.0 m/s. Both rocks land at the same time. How high is the cliff?




    2. I know some kinematics equations must be used



    3. I am stumped because I dont know the final velocity the height or the time just the difference in time between the two
     
  2. jcsd
  3. Dec 14, 2008 #2

    rl.bhat

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    What is your sign convention? Why the velocity is negative?
     
  4. Dec 14, 2008 #3
    i dont know what you mean by sign convention but velocity is negative because it is traveling downwards
     
  5. Dec 14, 2008 #4

    rl.bhat

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    In that case, what is the direction of acceleration due to gravity, g?
     
  6. Dec 14, 2008 #5
    -9.8
     
  7. Dec 14, 2008 #6
    d=d0+v*t+1/2at2
    d=d0+v*(t-.800s)+1/2a(t-.800s)2

    First rock,
    d0=0
    V=0
    a=9.81
    d=1/2at2

    Second rock,
    d0=0
    d=10m/s*(t-.800s)+1/2*9.81*(t-.800)2

    I used calculator to find when these to intersect.
    y1=1/2at2
    y2=10m/s*(t-.800s)+1/2*9.81*(t-.800)2
    Calc->Intersect
    I got x =2.26sec y = 25.02 m
     
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