# Homework Help: Dropping stone down the well.

1. Dec 19, 2012

### Confusedkid34

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

How deep is the well? Drop a stone and I took 16s to hit the ground level.

The air temperature is 20 degrees.

Acceleration is 9.8 m/s squared.

M=1kg

V1=0 m/s

Speed of sound= 343.8m/s squared
Find how deep the well is.
2. Relevant equations

D=t/v

I don't really know at all

Speed of sound.

3. The attempt at a solution

No ideas.

2. Dec 19, 2012

### SammyS

Staff Emeritus
Hello Confusedkid34. Welcome to PF!

Your obviously missing some words in your problem statement. What is it that takes 16 seconds to hit the ground?

To make all this clearer, please state the problem word for word as it was given to you.

3. Dec 19, 2012

### Staff: Mentor

Unless there's more to the problem statement than you've posted, only some of the given information is relevant.

Does the stone have a constant speed as it falls, or does it accelerate? What formula applies?

4. Dec 19, 2012

### Confusedkid34

The speed is constant, it takes 16s to hear the Rock hit the bottom.

So 16s for the rock to hit the bottom and for the noise to travel back.

It starts at rest. Both the sound and rock travel the same distance.

Speed of sound is 343.8 m/s

So time equals 16s. t=t1+t2

T1= time for the stone to hit the ground.

T2= time for sound to travel back.

So we need to solve for d. By finding out what t1 and t2 are.

Last edited: Dec 19, 2012
5. Dec 19, 2012

### SammyS

Staff Emeritus
How far does sound travel in a time of t2 ?

How far does the rock fall in a time of t1 ?

If t1 + t2 = 16, then express t2 in terms of t1 .

Put all of that together, solving for t1.

6. Dec 19, 2012

### Confusedkid34

The distance for t1 and t2 are the same.

But I don't understand how to do that?

Somewhere do I have to apply this in a quadratic ?

7. Dec 19, 2012

### Staff: Mentor

What happened to gravity? Your problem statement didn't mention anything about hearing the rock hit the bottom, only that it took 16 second to hit the bottom, no sound involved.

So really your problem statement should go something like:

An observer drops a stone from rest into a deep well. The stone falls under the influence of gravity. 16 seconds after the stone begins its fall the observer hears the sound of the rock hitting the bottom. How deep is the well?
Sounds like a plan. Can you write the expressions for each time?

8. Dec 19, 2012

### SammyS

Staff Emeritus
Please use the QUOTE button to indicate which post you are relying to.

It will help a lot.

I'll do this post, then let gneill finish up. (If he's willing.)

Answering them will help significantly in solving this problem.

9. Dec 19, 2012

### Confusedkid34

Your right. I didn't include all the information.

So we need an equation to solve for d. As well we need to figure out t1 and t2.

I have no idea how to do this and I could use some guidance.

10. Dec 19, 2012

### Staff: Mentor

Suppose you happened to know the distance d. How would you write an expression for the amount of time, call it t2, it takes for the sound to travel that far?