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 sound of frequency 780 ~Hz . What is the frequency that the operator hears in the echo reflected from the building. [Speed of sound is 340 ~m / s .]

Step-by-step solution

A drone fitted with siren as shown in the figure, Given, speed of drones, v_s=15 ~m / s , actual frequency, n₀=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= n₀ v v-v_s = 780 340 340-15 =816 ~Hz As we know, there is no change in the apparent frequency by the reflection from the building. Hence, the echo heard by observer is 816 ~Hz . So, the correct option is (3).