a) Sound waves are called mechanical waves because they are produced by the actual motion of the particles of a medium about their mean position.
The two practical applications of the reflection of sound are :
(i) In megaphones, to send sound waves in a particular direction.
(ii) In stethoscope, to listen to the heartbeats of a patient.
(b) Here, S = 125 m,
g = 10 m s^-2,
u = 0,
t = t1
Time taken by the stone to fall on the top of the pond.
Now, S = ut + 1/2 gt^2
125 = 0 + 1/2 x 10 x t1^2
t1 = 5 s
Time taken by sound to travel from the top of the pond to the ear
t2 = S/V = 125/340 = 0.37 s
Hence, total time
= 5 + 0.37
= 5.37 s