# physics

boy throws a penny into an empty well with an initial velocity of –2 m/s. He hears it hit the bottom 8 seconds later. About how far did the penny fall?

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1. t'=time to fall
t"=time for sound to return after bottom

t'+t"=8

depth=2*t'+1/2 g t'^2
depth=velocitysound*t"

set them equal (same depth)

2*t'+1/2 g t'^2=velocitysound*t"
2*t'+1/2 g t'^2=velocitysound*(8-t')

so solve for t', and then you can get depth.

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posted by bobpursley

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