A train approaching a railway crossing at a speed of 120 km/h sounds a whistle at frequency 640 Hz when it is 300 m away from the crossing. The speed of sound in air is 340 m/s . What will be the frequency heard by a person standing on a road perpendicular to the track through the crossing at a distance of 400 m from the crossing?

Question

A train approaching a railway crossing at a speed of 120 km/h sounds a whistle at frequency 640 Hz when it is 300 m away from the crossing. The speed of sound in air is 340 m/s . What will be the frequency heard by a person standing on a road perpendicular to the track through the crossing at a distance of 400 m from the crossing? (A) 680 Hz (B) 640 Hz (C) 720 Hz (D) 358 Hz

Tardigrade Admin · Accepted Answer

vs = 120 km/h = (100/3) m/s <img class="img-fluid question-image" alt="image" src="https://cdn.tardigrade.in/img/question/physics/0e5db50e6f6b1adea8ff29d9fd141d0a-.png" /> n = 640 Hz and v= 340 m/s n'= n ((v/v-vs cos θ)) = 640 ((340/340 - (100)3× (300)500) n' = 680 Hz

680 Hz

640 Hz

720 Hz

358 Hz

vs​=120km/h=3100​m/s n=640Hz and v=340m/s n′=n(v−vs​cosθv​) =640(340−3100​×500300​340​) n′=680Hz

$v_{s} = 120 km / h = \frac{100}{3} m / s$

$n = 640 Hz$ and $v = 340 m / s$
$n^{'} = n (\frac{v}{v - v _{s} cos θ})$
$= 640 (\frac{340}{340 - \frac{100}{3} \times \frac{300}{500}})$
$n^{'} = 680 Hz$