Doppler Effect Physics Example 3

Follow the full solution, then compare it with the other examples linked below.

Example 3

medium
A car horn emits 400 Hz400 \text{ Hz}. You drive toward the stationary car at 20 m/s20 \text{ m/s}. What frequency do you hear? Use vsound=340 m/sv_{\text{sound}} = 340 \text{ m/s}.

Solution

  1. 1
    When the observer moves toward a stationary source: f′=f×v+vovf' = f \times \frac{v + v_o}{v}.
  2. 2
    f′=400×340+20340=400×360340=400×1.059≈424 Hzf' = 400 \times \frac{340 + 20}{340} = 400 \times \frac{360}{340} = 400 \times 1.059 \approx 424 \text{ Hz}

Answer

f′≈424 Hzf' \approx 424 \text{ Hz}
The Doppler effect also occurs when the observer moves. An observer approaching a stationary source hears a higher frequency because they encounter wave crests more frequently.

About Doppler Effect

The change in the observed frequency (and wavelength) of a wave when the source and the observer are in relative motion.

Learn more about Doppler Effect →

More Doppler Effect Examples

Example 1 easy

An ambulance siren emits sound at [formula]. As the ambulance approaches you at [formula], what freq

Example 2 medium

A train whistle blows at [formula]. You are stationary and hear [formula]. Is the train approaching

Example 4 hard

Police radar emits microwaves at [formula]. The reflected signal from an approaching car has a frequ

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