(a) Ultrasound can be used in:
(i) Monitoring foetus during pregnancy.
(ii) Medical imaging of internal organs to check their size, structure and physiological functioning in humans.
(iii) Breaking kidney stones into smaller sizes to flush out in the urine.
Distance, d = 2800 m
Time, t = 40 sec
Velocity, v =?
Total distance travelled by sound = speed of sound × time
So, 2d = v × t
⇒ v = 2d / t
⇒ v = 2 × 2800 / 4
= 1400 m/sec