Two trains A and B are moving towards each other with speeds 72 kmh ⁻¹ and 36 kmh ⁻¹ respectively. The train A whistles at 640 ~Hz frequency. Before the trains meet, frequency of sound heard by a passenger in Train B is ( Speed of sound in air =340 ~ms ⁻¹ ).

Step-by-step solution

Apparent frequency due to doppler's shift when both observer O and source are approaching to each other, is given by apparent frequency, f^ =f ( v+v₀ v-v_s ) ...(i) Here, v_o= observers speed =36 ~km / h =36 5 18 ~m / s =10 ~m / s v_s= source speed =72 ~km / h =72 5 18 =20 ~m / s v = speed of sound =340 ~m / s Hence, by eq. (i) apparent frequency, f^ =640 ( 340+10 340-20 )= 640 350 320 =700 ~Hz