speed of sound in air

oceanflavored22

i'm doing extra problems in my textbook but one of them baffles me...

a hiker shouts toward a vertical cliff 685 m away. the echo is heard 4 s later
what is the speed of sound of the hiker's voice in air?

i know to calculate this you do velocity=distance/time.
and i did velocity=685/4 and got 171.

but in the answers, the book says that you have to do velocity=685/2

i dont know how they got 2 s, when the question clearly states it's 4.
is it just a typo? or do you have to halve the time when it's air?

thanks for anyone who can help me with this conundum

brewnog

It's an echo!

Any cogs whirring....? Think about where the sound has to travel.

(I remember puzzling over exactly the same question when I was at school!)

oceanflavored22

uhhhhmmmmmm......no.
i dont think my physics teacher ever mentioned echos.
could you explain this to me?
thanks so much. 'preciate it. alottttt.

mace2

if you are 10m from a wall and you throw a ball at it, then catch it, how many meters has the ball travelled?

oceanflavored22

none? because the ball travels 10 m to the wall, but then comes back 10 m. ok...so i think i get of get what brewnog was saying about the echos. thanks

mace2

it travels 10m to the wall, and 10m back. therefore 20m travelled.

same thing with the sound. travels 685m there, and 658m back.

685+685=1370m.
1370m/4s = 685m/2s.
get it?

AkashBhatt

Hey Ocean...u got ur ans in ur question.
As u knw v=2d/t
v=2X685/4
Therefore ur text gave tht V=685/2
That is the right ans.

gneill

You seem to have some intermittent keys on your keyboard. Better get it checked out.

AkashBhatt

What does that mean?
Please do reply....

mstud

http://dictionary.reference.com/browse/intermittent

I think he's refferring to the lacking letters of your answer... we always prefer to guess the best, so we think you pressed the keys and they didn't respond rather than that you didn't touch them at all.

Refer to the posting guidelines for more info, you seem to have overlooked one point I think is mentioned there: Don't use uneccessary abbreviations...

AkashBhatt

Oh....I am really sorry for that i will take care next time...
thank you!