# Kinematics, rock falling off cliff

• 777lov3r
In summary: Using this information, we can set up the following equation:3.4 = [itex]t_1[/tex] + [itex]t_2[/tex]And using the formulas for [itex]D_1[/tex] and [itex]D_2[/tex], we can write:[itex]D_1 = \frac{1}{2}(-9.8m/s^2)(t_1)^2[/tex][itex]D_2 = 340m/s \cdot t_2[/tex]Since [itex]D_1=D_2[/tex], we can set them equal to
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.

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.

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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

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.

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

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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)

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

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]

## 1. What is kinematics?

Kinematics is the branch of physics that studies the motion of objects, including the forces and energy involved in that motion.

## 2. How does kinematics relate to a rock falling off a cliff?

Kinematics can be applied to analyze the motion of a rock falling off a cliff, including factors such as its initial velocity, acceleration due to gravity, and the distance it falls.

## 3. What is the equation for calculating the velocity of a falling rock?

The equation for calculating the velocity of a falling rock is v = u + gt, where v is the final velocity, u is the initial velocity, g is the acceleration due to gravity, and t is the time.

## 4. Can kinematics be used to predict the path of a falling rock?

Yes, kinematics can be used to predict the path of a falling rock by analyzing its velocity and acceleration at different points in time.

## 5. How does air resistance affect the motion of a falling rock?

Air resistance can slow down the motion of a falling rock, which can impact the distance it falls and its final velocity. Kinematics equations can be modified to take into account the effects of air resistance.

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