Question

the siren on a ambulance has a frequency of 840Hz . The ambulance passes away from you at a speed of 28m/s in the atmosphere temperature of 20°C . the speed of your motorcycle against the ambulance is 30m/s . what will be the apparent frequency you will hear ?​

Answer 1

Given info : the siren on a ambulance has a frequency of 840Hz . The ambulance passes away from you at a speed of 28m/s. the speed of your motorcycle against the ambulance is 30m/s.

To find : the apparent frequency you will hear ...

solution : here we have to use the concept of Doppler's effect.

when source and observer both are moving away from each other.

apparent frequency, $f'=f_0\left(\frac{v+v_0}{v-v_s}\right)$

where v is the speed of sound wave, f₀ is the frequency of the source.

v₀ is the speed of observer and $v_s$ is the speed of source.

here, v = 343 m/s at 20° C

v₀  = 30 m/s

f₀  = 840 Hz

$v_s$ = 28 m/s

now, f' = 840[(343 + 30)/(343 - 28)] = 994.67 Hz

therefore the apparent frequency you will hear is 994.67 Hz