Question

A person standing between  two vertical cliffs and 640 metres away from the nearest cliff shouted. He heard the first echo after 4 sec and the second echo after 3 sec. Calculate :(i) the velocity of sound in air(ii) distance between the two cliffs

Answer 1

vel=1280/4=320m/sec

2*d/3=320
d=480m
Distance between the 2 cliffs = 640+480=1120m

Answer 2

Given: Distance from nearest pillar = 640 m
time taken to hear the echo = 4 sec
we know ,
velocity = Distance / time
            = 2 * Distance /time
(note:- (2 * Distance)  because Echo travels twice the Distance )
            = (640* 2 )/ 4
            =320( m / s )
 (i) velocity of sound = 320 ( m/s )
now (ii)
Distance between Person & Piller 2
we know ,
   velocity = Distance / time
   Distance = Velocity * time
(note:- (2 * Distance) because Echo travels twice the Distance )
 
2*Distance = velocity * time
2*Distance = 320 * 3
  Distance  = ( 320 * 3 ) / 2
                 = 480 m
Distance between 2 pillars = 640 + 480
                                       =1120 m
(ii) 1120 m