posted by Pat on .
A boy drops a 0.10 kg stone down a 150 m well and listens for the echo. The air temperature is 20°C. How long after the stone is dropped will the boy hear the echo?
The speed of sound through air at 20°C is v = 343 m/s.
Downward motion of the stone:
h =g•t1²/2, t1 =sqrt(2•h/g) = sqrt(2•150/9.8) = 5.5 s.
Upward motion of sound:
h = v•t2.
t2 = h/v = 150/343 =0.44s.
t =t1+t2 =5.5 +0.44 =5.94 s.