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Homework Help: Kinematics, rock falling off cliff

  1. Sep 29, 2010 #1
    so im kind of stumped on this one here.... A rock is dropped from a sea cliff and the sound of it striking the ocrean is heard 3.4 seconds later. If the speed of sound is 340m/s, how high is the cliff?

    - now i got some variables, v1=0 m/s
    a= -9.80m/s^2
    delta d= ?

    i got thinking that the total time it takes for the rock to fall then the sound to come back at you would be 3.4 seconds in total, so i dont know how to seperate the time it takes the rock to fall from the time it takes the sound to actually get back to you.

    by using the 3.4 seconds, i tried to get a distance which was off, at 113 m, but then realized that i need to find out how long it takes for the rock alone to fall, without the sound. i cant find a reasonable way to get this number.

    please help!?
  2. jcsd
  3. Sep 29, 2010 #2
    Forget the rock. Just focus on the sound. You have a time it takes for the sound to travel a distance, and the velocity of that sound. Now take a look at your formula for speed.
    Last edited: Sep 29, 2010
  4. Sep 30, 2010 #3
    so I attempted this by saying 3.4s=t(rock falls)+t(sound back), then solved and got 2 answers, which were 112.2 m and -11908 m, and I tried seeing if the 112.2 m made any sense when I plugged It in, but it would take the rock more than 3.4 s to reach the bottom, so 112.2 m is impossible...any suggestions on the right equation to use?:s
  5. Sep 30, 2010 #4
    You can completely ignore the rock. You have the speed of sound, and the time it takes the noise to travel the height of the cliff.

    Your basic speed formula:
    [tex]S=\frac{D_{Final}-D_{Initial}}{\Delta Time}[/tex]
    Last edited: Sep 30, 2010
  6. Oct 5, 2010 #5
    but it's saying that the timer starts when the rock falls off the cliff of an unknown distance then sound travels back to u..in 3.4s, so would u use a formula 3.4=t(rock falls)+t(sound)?:)
  7. Oct 6, 2010 #6
    Oh, sorry. I misread the question...and I have no clue how to keep going.
  8. Oct 6, 2010 #7
    You started off right; 3.4s is the amount if time it takes the rock to fall from the cliff to the ocean ([itex]t_1[/tex]) plus the time it takes for the sound of the rock hitting the ocean to be heard from the top of the cliff ([itex]t_2[/tex]).

    What is the formula for the distance the rock falls (use [itex]t_1[/tex])?
    Call this [itex]D_1[/tex]

    What is the formula for the distance the sound of the rock hitting the ocean travels (use [itex]t_2[/tex])?
    Call this [itex]D_2[/tex]

    We know 2 things:
    1) [itex]t_1 + t_2 = 3.4 sec[/tex]
    2) [itex]D_1 = D_2[/tex]
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