Question

If the siren is at rest and the observe is moving away from the siren at 30 m/s then present frequency does the observe hear __________(Hz).

Answer 1

Given that,

Speed of observer = 30 m/s

Suppose,  Source frequency is 800 Hz.

Siren is at rest. so the speed of source = 0

We need to calculate the frequency hear by observer

Using formula of apparent frequency

$f'=\dfrac{v+v_{0}}{v-v'}\times f$

Where, f = frequency of source

v = speed of sound

v'= speed of observer

$v_{0}$ =speed of source

Put the value into the formula

$f'=\dfrac{330+30}{330-0}\times800$

$f'=872.7\ Hz$

Hence, The frequency hear by observer is 872.7 Hz.