# Sound Question

#### green11

1. The problem statement, all variables and given/known data

A stone is dropped into a well. The sound of the splash is heard 2.30 s later. What is the depth of the well?

2. Relevant equations

g = -9.8 m/s^2
Vsound in air = 343 m/s

3. The attempt at a solution

I'm not really sure how to start. I know that the time it takes to drop down + the time it takes the sound to go back up must = 2.3 seconds

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#### green11

<< two threads merged by berkeman >>

Last edited by a moderator:

#### jgens

Gold Member
Well, write out what else you know. What do you know about the distance the stone must fall and the distance the sound waves must travel? What about the time it takes the rock to fall a distance h?

#### jgens

Gold Member
For the future, please do not post your homework questions in multiple forums.

#### diazona

Homework Helper
I know that the time it takes to drop down + the time it takes the sound to go back up must = 2.3 seconds
Try writing an equation for that. I'd suggest using variables like $t_\text{drop}$ $t_\text{sound}$.

#### green11

For the future, please do not post your homework questions in multiple forums.
I'm sorry. I posted it in here first then realized it might better belong in the other one.

#### green11

Try writing an equation for that. I'd suggest using variables like $t_\text{drop}$ $t_\text{sound}$.

$t_\text{drop}$ + $t_\text{sound}$ = 2.3 s

Like that?

#### green11

Well, write out what else you know. What do you know about the distance the stone must fall and the distance the sound waves must travel? What about the time it takes the rock to fall a distance h?
The distance the stone falls = The distance the sound waves travel

h = (1/2)(-9.8)t^2

Rearranging this, I got that t = sqrt(-h/4.9)

#### jgens

Gold Member

trock + tsound = 2.3

You have an expression trock. Knowing that the sound waves must travel a distance h with velocity v = 343 m/s can you complete the equation?

#### green11

trock + tsound = 2.3

You have an expression trock. Knowing that the sound waves must travel a distance h with velocity v = 343 m/s can you complete the equation?
Would that just be

sqrt(-h/4.9) + 343h = 2.3 s ??

#### jgens

Gold Member
Not quite. Since you're only worried about magnitudes here, we don't need to worry about h being positive or negative, so your equation should read,

2.3 s = sqrt(h/g) + h/vs

Remember t != v(h), t = h/v or h = vt.

Thank you!

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