Question

Standing on the sidewalk, you listen to the horn of a passing car. As the car passes, the frequency of the sound changes from high to low in a continuous manner; that is, there is no abrupt change in the perceived frequency. This occurs because a) the pitch of the sound of the horn changes continuously. b) the intensity of the observed sound changes continuously. c) you are not standing directly in the path of the moving car. d) of all of the above reasons.

Short answer

Answer: The Doppler effect is the reason behind the change in frequency of the sound heard when a moving car with a horn passes by a person standing on the sidewalk.

Step-by-step solution

Understand the Doppler Effect

The change in frequency of the observed sound when the source of the sound is moving in relation to the observer is called the Doppler effect. This phenomenon occurs for any type of wave, including sound waves and light waves. In this case, the moving car with a horn is the source of the sound, and the stationary person on the sidewalk is the observer.

Analyze the effect of the Doppler effect on the frequency

Based on the Doppler effect, we can conclude that when the moving car is approaching the person, the observed frequency of the sound wave increases (the sound appears higher pitched). Conversely, when the car is moving away from the person, the observed frequency decreases (the sound appears lower pitched). The change in the perceived frequency is a result of the relative motion between the source and the observer, and not due to any inherent change in the sound itself.

Evaluate the given options

Now, let's evaluate the given options: a) The pitch of the sound of the horn changes continuously: This is incorrect because the change in the observed frequency is due to the Doppler effect, not an inherent change in the pitch of the sound produced by the horn. b) The intensity of the observed sound changes continuously: This is incorrect because the intensity of the sound may change, but it is not directly related to the change in perceived frequency. c) You are not standing directly in the path of the moving car: This is also incorrect because the location of the observer does not affect the change in the observed frequency due to the Doppler effect. d) Of all the above reasons: This is incorrect because none of the options provided are the correct reason for the change in the observed frequency. The correct reason is the Doppler effect, which is not listed among the options. #Conclusion# In conclusion, none of the options provided in the exercise are the correct reason for the continuous change in the frequency of the sound heard as the car passes by the person. The correct reason is the Doppler effect.

Key concepts

Sound Waves

When you hear the horn of a car, what you are actually detecting are sound waves. These waves are created by vibrations and they travel through the air to reach your ears. Sound waves are longitudinal, which means they compress and expand as they move.

Think of sound waves like ripples in a pond when a stone is dropped. They spread outwards from the source (the horn in this case) until they reach the listener. Here’s a breakdown of how sound waves work:

• Sound waves have frequency, which determines the pitch of the sound you hear. Higher frequencies sound higher pitched, and lower frequencies sound lower pitched.
• The speed of sound in air is approximately 343 meters per second. This means, depending on distance, there can be a delay before you hear the sound after it is made.
• Sound waves can bounce off surfaces (reflection), be absorbed by materials (absorption), or pass into different materials (transmission).

Understanding these properties helps explain phenomena like echoes and why you might hear a sound differently depending on your environment.

Frequency Change

The concept of frequency change is central to understanding the Doppler Effect. Frequency itself refers to how many waves pass a point in a second and is measured in Hertz (Hz).

When the source of the sound waves (like a car horn) moves relative to an observer, the frequency of the waves changes. Here's how this frequency change unfolds:

• As the car approaches, sound waves get compressed because each subsequent wave is emitted from closer than the last, resulting in a higher frequency or pitch.
• Conversely, as the car moves away, the waves stretch out, creating a lower frequency or pitch as each subsequent wave is emitted from a position further away.
• This alteration in frequency is not due to any change in the sound being emitted by the car horn but is purely due to the relative motion between the sound source and the observer.

This phenomenon is what makes the horn of a moving car appear to change pitch as it moves past you, from high to low.

Observer Perception

Our perception of sound, especially when in motion, is influenced by the Doppler Effect. For an observer standing still, the changes in frequency as a car approaches and then moves away can be clearly identified.

Here’s how the observer perceives sound:

• As the car moves closer, the waves reach the observer more frequently, increasing the perceived pitch. It seems like the sound is getting sharper.
• Once the car passes by, the observer suddenly perceives a drop in pitch as the frequency decreases. It’s like the sound is slowing down or stretching out.
• This perception is continuous and happens because our ears naturally detect changes in sound frequency, allowing us to distinguish the direction and speed of moving sound sources.

Our brains automatically process these changes, so even if we consciously don't think about it, our ability to perceive these frequency changes helps us navigate the world and react to moving objects.