Sound and kinematic problem solving

[anna sung]

Homework Statement

a man dropped a stone into a can that is 120m deep. if the temperature of the air is 20degree celsius. how long would it take for a man to hear the sound when the stone hit the can?

Homework Equations

d=Vi*t + 1/2*a*t^2

The Attempt at a Solution

since i know that the initial velocity is 0.0m/s
i canceled out the part vi*t
and solved for time ----> t= $$\sqrt{}2(d)/a$$
which is t= $$\sqrt{}2(120m)/9.81m/s^2$$= 7.2s

is this correct.? I am having a doubt on my answer because i didnt use the temperature at all.

 

[p21bass]

I'm not sure how you got 7.2s from that equation - maybe you entered in incorrectly into your calculator?

And the temperature is really only used as a reference in regards to the speed of sound. So, you have the time it takes the stone to hit the bottom of the can, and then the time for the sound to travel back to the man.

 

[anna sung]

I'm not sure how you got 7.2s from that equation - maybe you entered in incorrectly into your calculator?

And the temperature is really only used as a reference in regards to the speed of sound. So, you have the time it takes the stone to hit the bottom of the can, and then the time for the sound to travel back to the man.

wait. sorry i got wrong number from the calculator but the equation is right.right?
and also so are you saying that i had to use the temperature. and find thae speed of the time and add it with the time i got from the kinematic equation or just what i did was the end?

 

[gneill]

I'm not sure how you got 7.2s from that equation - maybe you entered in incorrectly into your calculator?

And the temperature is really only used as a reference in regards to the speed of sound. So, you have the time it takes the stone to hit the bottom of the can, and then the time for the sound to travel back to the man.

Unless, of course, the man hears the sound that travels up through the walls of the can first!

That's a *big* can!

 

[anna sung]

Unless, of course, the man hears the sound that travels up through the walls of the can first!

That's a *big* can!

what? wait. I am still confused so you mean the equation i did was the end. or i had to do add the time i got using the temperature?

 

[gneill]

There are two time periods involved. The first (which you've already addressed) is the time it takes for the stone to reach the bottom of the can. The second period is the time it takes for the sound from the impact to travel back up to the man. This second period involves the speed of sound in air which is temperature dependent (but not by too much unless the temperatures get really far from room temp). Simply look up the speed of sound at that temperature.

 

[anna sung]

There are two time periods involved. The first (which you've already addressed) is the time it takes for the stone to reach the bottom of the can. The second period is the time it takes for the sound from the impact to travel back up to the man. This second period involves the speed of sound in air which is temperature dependent (but not by too much unless the temperatures get really far from room temp). Simply look up the speed of sound at that temperature.

no!so that means in the end you were supposed to add both of the times and that is your final answer? cause this question was on my test i stopped at the kinematic equation.

 

[p21bass]

Yes, that's how that problem is solved.

 

[anna sung]

Yes, that's how that problem is solved.

ah man..at lest i did the half correctly . thank you so much for help! it was a great help!