Question

The driver of a car hears the siren of an ambulance that is moving away from her. If the actual frequency of the siren is 2000 hertz, which frequency was heard by the driver?

A.
1900 Hz

B.
2000 Hz

C.
2100 Hz

D.
4000 Hz

Answer 1

To determine the frequency heard by the driver of the car when the ambulance is moving away from her, we can use the Doppler effect formula for sound. The formula used when the source is moving away from the observer is:

\[
f' = f \cdot \frac{v}{v + v_s}
\]

where:
- \( f' \) is the frequency heard by the observer (the driver),
- \( f \) is the actual frequency of the source (the siren frequency),
- \( v \) is the speed of sound in air (approximately \( 343 \, \text{m/s} \) at room temperature),
- \( v_s \) is the speed of the source (the ambulance).

Since we do not have the value for the speed of the ambulance, we cannot directly calculate \( f' \) without that information. However, we can analyze the given choices based on the known behavior of the Doppler effect.

1. If the ambulance is moving away, the frequency heard by the driver will be less than the actual frequency of the siren (2000 Hz).
2. The options given are:
- A. 1900 Hz
- B. 2000 Hz
- C. 2100 Hz
- D. 4000 Hz

Since the ambulance is moving away from the driver, the frequency that the driver hears must be lower than 2000 Hz.

Therefore, the correct answer is:

**A. 1900 Hz**