A person driving car at a constant speed of 15 m/s is approaching a vertical wall. The person notices a change of 40 Hz in the frequency of his car's horn upon reflection from the wall. The frequency of horn is ______ Hz.
Physics · Circular Motion
JEE Mains 6 April 2023 Shift 1 Morning Shift

Question

A person driving car at a constant speed of 15 m/s is approaching a vertical wall. The person notices a change of 40 Hz in the frequency of his car's horn upon reflection from the wall. The frequency of horn is ______ Hz.

Physics · Circular Motion

JEE Mains 6 April 2023 Shift 1 Morning Shift

G1 Physics · Accepted Answer

The driver hears the reflected sound from the wall. There are two Doppler shifts: first when the wall receives the horn sound and second when the driver receives the reflected sound. For a driver moving toward the stationary wall with speed u, the reflected frequency heard by the driver is f' = f(v + u)/(v − u) Given: u = 15 m s⁻¹ v = 330 m s⁻¹ f' − f = 40 Hz So, 40 = f[(330+15)/(330−15) − 1] 40 = f[(345/315) − 1] = f(30/315) Therefore, f = 40×315/30 = 420 Hz Final value: 420