Posted by **Sara** on Tuesday, December 15, 2009 at 2:08pm.

suppose that you wish to determine how far away from your position lightning has struck. the thunder that results from lightning expands outward in all directions at a constant speed (you may assume 343 m/s at 20C). when counting seconds between the time you see the lightning and the time you hear the thunder, what should you divide by (to three significant figures) to determine the number of miles away the lightning has struck?

- Physics -
**Sara**, Tuesday, December 15, 2009 at 2:12pm
i know that i could multiply by the number of second to figure out how many miles it traveled for that time but i don't know where to go from there.

- Physics -
**MathMate**, Tuesday, December 15, 2009 at 2:51pm
First convert the speed of sound (343m/s) to miles per second.

For all practical purposes, we can consider the time it takes the lightning to reach the observer as negligible.

So the number of seconds the observer counted, *multiplied* by the speed of sound (in miles / second) will be the distance, in miles.

If the number of seconds is to be *divided*, then it should be divided by the reciprocal of the speed of sound, namely in seconds/mile.

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