Question

An unbelievably LOUD car alarm at 4:12 AM at the intersection of Jewel Avenue and 150th Street sends out a pure tone at 400 s-1. The speed of sound is 343 m/s. What frequency is heard from:

a) a car racing TOWARD it at 50 m/s (112 mph)

b) a car racing AWAY from it at 50 m/s (112 mph)

Answer 1

Part A.

When observer is moving towards stationary source, then Using Doppler's effect

f1 = f0*(V + Vo)/V

Vo = Speed of observer = 50 m/sec

f0 = Original frequency = 400 s^-1 = 400 Hz

V = Speed of sound = 343 m/sec

So, observed frequency will be

f1 = 400*(343 + 50)/343

f1 = 458.3 Hz

B.

When observer is moving away from stationary source, then Using Doppler's effect

f2 = f0*(V - Vo)/V

Vo = Speed of observer = 50 m/sec

So,

f2 = 400*(343 - 50)/343

f2 = 341.7 Hz

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