# [SOLVED] speed of light and sound problem

• Jun 8th 2010, 09:18 AM
clockworks204
[SOLVED] speed of light and sound problem
The speed of light is 3.0×108 m/s and the speed of sound is 3.4×102 m/s. Find the value of the integer N in the following statement: If you start counting seconds when you see something happen and stop when you hear it happen, then the event is about 1km away for every N seconds counted.

I'm a little confused on how to work this. I think that I should start by converting m/s into km/s, but after that I've got nothing! Thanks in advance!
• Jun 8th 2010, 09:58 AM
masters
Quote:

Originally Posted by clockworks204
The speed of light is 3.0×10^8 m/s and the speed of sound is 3.4×10^2 m/s. Find the value of the integer N in the following statement: If you start counting seconds when you see something happen and stop when you hear it happen, then the event is about 1km away for every N seconds counted.

I'm a little confused on how to work this. I think that I should start by converting m/s into km/s, but after that I've got nothing! Thanks in advance!

Hi clockworks204,

Sound travels about 1 km in 3 seconds. (3 times 3.4^2 = 1029)

So, N = 3.
• Jun 8th 2010, 10:15 AM
clockworks204
Soo much easier than I made it out to be. Thank you!
• Jun 8th 2010, 10:27 AM
Unknown008
Since light travels very fast (the fastest 'thing' known to now), we can say that you see something instantaneously. Then, you only consider the speed of sound.

Use the formula :

$Distance = Speed \times Time$

You have:

$1000 m = 3.4 \times10^2 t$

$t = 2.94 s$

So, you say that for sound to travel 1 km, it takes 3 seconds (integer), and hence, N = 3.
• Jun 8th 2010, 10:28 AM
Unknown008
Sorry, duplicate post... If a mod could delete that, it would be nice.