Physics
posted by Kristina .
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?

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 
Try some of the Physics Tutorials:
http://search.yahoo.com/search?fr=mcafee&p=Physics+tutorial
Sra
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