Questão de Natural Sciences and their Technologies #019e354a
Uma ambulância em alta velocidade com a sirene ligada desloca-se em direção a um radar operado por uma pessoa. O radar emite ondas de rádio com frequência f0 que são refletidas pela dianteira da ambulância, retornando para o detector com frequência fr. A percepção do operador do radar, em relação ao som emitido pela sirene, é de que este se altera à medida que a ambulância se aproxima ou se afasta.
Durante a aproximação, como o operador percebe o som da sirene e qual é a relação entre as frequências fr e f0 medidas pelo radar?
Lower than the sound emitted and fr < f0.
Higher pitch than the sound emitted and fr < f0.
Higher in pitch than the sound emitted and fr = f0.
Higher pitch than the sound emitted and fr > f0.
More serious than the sound emitted and fr > f0.
Explicação Oficial / Explicações da Comunidade
Analysis of the Doppler Effect for Sound and Radio Waves
The correct answer is option A. D) Higher pitch than the emitted sound and fᵣ > f₀.
1. The Physical Principle: Doppler Effect
Both parts of the question are explained by Doppler effect, which describes the change in the perceived frequency of a wave when there is relative motion between the emitting source and the observer.
- Approximation: When the source and the observer approach each other, the perceived frequency is bigger (higher) than the emitted frequency. * Absence: When the source and the observer move away from each other, the perceived frequency is... smaller (lower) than the emitted frequency.
2. Analysis of the Siren Sound
In this part, the source is... ambulance siren (in motion) and the observer is the person with the radar (at rest).
- Situation: The ambulance is... approaching from the operator. * Application of the Doppler Effect: Due to the proximity, the operator will perceive the sound at a frequency... bigger than the actual frequency emitted by the siren. * Human Perception: For sound, a higher frequency is perceived as a louder sound. acute.
Therefore, the first part of the answer is that sound is perceived as sharper.
3. Analysis of Radar Radio Waves
In this part, the phenomenon occurs in two stages, resulting in a double Doppler effect:
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Step 1: Radar (source) to Ambulance (observer) * The radar emits a wave with a frequency f₀. * The ambulance, when it approach From the wave source, it acts as a moving observer. It receives radio waves with a frequency f' which is bigger that f₀ (f' > f₀).
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Step 2: Ambulance (now a reflective source) to Radar (observer) * The ambulance reflects these waves. In doing so, it becomes a moving source, emitting waves with the frequency f' that it received. * As the ambulance (reflector source) is positioned approaching From the radar (observer), the radar will detect these reflected waves with a frequency fᵣ which is still bigger that f'.
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Final Report: If fᵣ > f' ef' > f₀, then, as a direct consequence, fᵣ > f₀.
4. Conclusion
Combining the two analyses:
- The sound of the siren is perceived as sharper because the perceived frequency is higher due to proximity. 2. The frequency of the reflected radio wave (fᵣ) is bigger that the emitted frequency (f₀) is due to the double Doppler effect during the approximation.
Therefore, the only alternative that correctly combines the two conclusions is: D.