# Physics

At a height of 15 meters above the surface of a lake, a sound pulse is generated. The echo from the bottom of the lake returns to the point of origin 0.150 s later. The air and water temperatures are 20°C. How deep is the lake?

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1. look up speed of sound in water at 20C
call it Cw
look up speed of sound in air at 20C
call it Ca
then distance in water = 2d
distance in air = 30
time in water = t
time in air = (.15 -t)
then
time in air = 30/Ca = .15 - t
solve for t
then
Cw t = 2 d

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posted by Damon
2. Try some of the Physics Tutorials:

http://search.yahoo.com/search?fr=mcafee&p=Physics+tutorial

Sra

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