Sound Waves dropping a rock into a well

[newfie88]

Homework Statement

When you drop a rock into a well, you hear the splash 1.5 sec later. How deep is the well?

Homework Equations

The Attempt at a Solution

Speed of sound 343m/s
x=(343m/s)(1.5)=514.5

 

[hage567]

No. The 1.5 seconds is the time it takes the rock to drop to the water plus the time it takes the sound to come back up the well to your ear.

Try again.

 

[newfie88]

do i need to take half of the time...Also is the answer 11 m.

 

[hage567]

do i need to take half of the time...Also is the answer 11 m.

No. The rock doesn't fall at the same speed that the sound wave travels. The time for each direction will not be equal.

Break it up into two parts. You can express the distance the rock falls in terms of the time it takes to reach the water (call that t₁). Then express the distance in terms of the velocity of the sound wave as it goes back up the well (call that time t₂).

You were given the total time, so you can relate these equations to each other.

 

[newfie88]

Im sorry i don't really get this whole physics thing...does gravity have anything to do with this question...and the answer in back of the book is 11 m but no matter what i do icant seem to get that answer

 

[hage567]

Im sorry i don't really get this whole physics thing...does gravity have anything to do with this question...and the answer in back of the book is 11 m but no matter what i do icant seem to get that answer

Yes gravity is involved because the rock is falling vertically.
Start by doing this: Look up your kinematic equations. Show me the equation for distance (y, say) in terms of g, and t1 (the time it takes the rock to fall the distance y). You know the initial velocity of the rock is zero.

If you show your work I can more easily point out what you are doing wrong.