1)

A drone fitted with siren in flying directly away from the drone operator and towards a distant building at a speed of 15 m/s . The siren produces the sound of frequency 780Hz. What is the frequency that the  operator hears in the echo refected from the building,[ speed of sound is 340 m/s]

A) 766 Hz

B) 800 Hz

C) 816Hz

D) 840Hz

Option C

### Explanation:

A drone fitted with a siren  as shown in the figure,

Given, speed of drones , $v_{s}=15 m/s$ , actual  frequency , $n_{0}=780 Hz$ and speed of sound , v=340 m/s

As source  is moving towards  the building, so the apparent frequency reflected by the building is

$n= \frac{n_{0}v}{v-v_{s}}= \frac{780 \times 340}{340-15}=816 Hz$

As, we know there is no change in the apparent  frequency by the reflection from the building Hence,

the echo heard by the observer is 816 Hz.

So, the correct option is (c)