Car P moves at 20 m s⁻¹ and sounds a 400 Hz horn. Car Q is behind P, moving in the same direction at 40 m s⁻¹. Take sound speed 360 m s⁻¹. The frequency heard in Q is approximately:
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Car P moves at 20 m s⁻¹ and sounds a 400 Hz horn. Car Q is behind P, moving in the same direction at 40 m s⁻¹. Take sound speed 360 m s⁻¹. The frequency heard in Q is approximately:

G1 Physics · Accepted Answer

The observer Q moves toward the source wavefronts while source P moves away from Q. Using the Doppler formula with the appropriate signs: f' = f(v + v(o))/(v + v(s)) = 400(360+40)/(360+20) ≈ 421 Hz.