Kinematics, rock falling off cliff

[777lov3r]

so I am 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
time(total)=3.4s
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 don't know how to separate 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 can't find a reasonable way to get this number.

please help!?

 

[Lancelot59]

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.

 

[777lov3r]

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

 

[Lancelot59]

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:
$$S=\frac{D_{Final}-D_{Initial}}{\Delta Time}$$

 

[777lov3r]

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)

 

[Lancelot59]

Oh, sorry. I misread the question...and I have no clue how to keep going.

 

[zgozvrm]

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]