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Sound Question

  • Thread starter green11
  • Start date
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
 
<< two threads merged by berkeman >>
 
Last edited by a moderator:

jgens

Gold Member
1,577
49
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
1,577
49
For the future, please do not post your homework questions in multiple forums.
 

diazona

Homework Helper
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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 [itex]t_\text{drop}[/itex] [itex]t_\text{sound}[/itex].
 
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.
 
Try writing an equation for that. I'd suggest using variables like [itex]t_\text{drop}[/itex] [itex]t_\text{sound}[/itex].

[itex]t_\text{drop}[/itex] + [itex]t_\text{sound}[/itex] = 2.3 s

Like that?
 
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

Also, I know that the equation for free fall distance is:
h = (1/2)(-9.8)t^2

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

jgens

Gold Member
1,577
49
So, from your other thread you said,

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?
 
So, from your other thread you said,

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
1,577
49
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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