Problem 42 - Pearson Physics

Question

If you move away from a stationary source of sound, is the frequency you hear greater than, less than, or equal to the frequency produced by the source? Explain.

Step-by-Step Solution

Answer

The frequency heard is less than the frequency produced by the source.

Step 1: Understand the Doppler Effect

The Doppler Effect describes the change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source. This is noticeable in sound waves when either the source, the observer, or both, are moving.

Step 2: Analyze the Movement

Since you are moving away from a stationary sound source, it means the distance between you and the source is increasing over time.

Step 3: Determine the Effect on Frequency

According to the Doppler Effect, when the distance between the listener and the source increases, the frequency heard by the observer is lower than the frequency emitted by the source. This happens because the sound waves reach the observer less frequently as the distance increases.

Key Concepts

Frequency

Frequency refers to the number of times a wave oscillates or repeats itself within a given period. It is measured in hertz (Hz), which counts the number of cycles per second. In the context of sound, frequency is related to the pitch of the sound we hear – higher frequencies create higher pitch sounds, while lower frequencies are heard as lower-pitched sounds.

When considering the Doppler Effect, frequency becomes crucial because it determines how sound is perceived when either the source or the observer is moving. For instance, if you hear a police car siren while walking towards it, the sound appears higher in frequency as you approach, due to the sound waves being compressed. Conversely, as you move away, the frequency reduces, making the sound appear lower in pitch.

Sound Waves

Sound waves are longitudinal waves that travel through a medium like air, water, or solid materials. They are produced by vibrations and propagate by compressing and rarefying particles in the medium.

• Compression: Part of the sound wave where particles are close together.
• Rarefaction: Part of the sound wave where particles are spread apart.

The speed and behavior of sound waves depend on various factors, such as the medium and temperature. In relation to the Doppler Effect, these waves change in apparent frequency depending on relative motion. By understanding how sound waves function, we can grasp why moving relative to a sound source influences the sound we hear.

Observer Movement

Observer movement is a key element in understanding the Doppler Effect. When an observer moves towards or away from a sound source, the relative speed alters the frequency of the sound they perceive.

In the given scenario where an observer moves away from a stationary sound source, the sound waves get stretched out. It means that each successive wave takes a bit longer to reach them compared to the previous one.

As a result, the frequency decreases, leading the observer to hear a sound of lower pitch than that of the source. This principle is fundamental in various practical applications, such as radar and echolocation technologies, where understanding wave interactions with moving observers can enhance precision.

• Moving towards the source: Higher frequency perceived.
• Moving away from the source: Lower frequency perceived.

Understanding how an observer's movement affects sound perception is vital in fields ranging from acoustics to astrophysics.