Question

A racecar is traveling around a track. Under what circumstance would you hear the highest pitch?

A) As the car is moving slowly toward you.
B) As the car is moving quickly toward you.
C) As the car is moving slowly away from you.
D) As the car is moving quickly away from you.

Answer 1

B.

Due to the Doppler Effect the frequency increases as an object approaches. As the car is moving quickly toward you, the sound will appear the highest pitch.

Answer 2

• Option B) As the car is moving quickly toward you.

Step-by-step explanation:

The higher the frequency of the sound wave the higher the pitch. So, the sound with the highest frequency will have the highest pitch.

When the source of a sound (the car) has a relative motion to an observer (you) there is an apparent change in the frequency of the sound wave. This is known as the Doppler effect.

As the source of the sound is moving toward the observer, the sound waves will shorthen their wave length, so, since the inverse relation between the wave length and the frequency, the result is an apparent increase of the frequency of the sound.

The faster the source is moving toward you (the observer) the greater the effect (increase of the frequency) and so the higher the pitch. This is what the option B) describes: as the car is moving quickly toward you, you would hear the highest pitch.

The opposite happens when the source of the sound moves away from the observer: the pitch lowers.

The Doppler effect is also observed with the light waves: when the light source is moving toward the observer the apparent change in the frequency implies a blue shift, when the source is moving away of the observer the apparent increase of the frequency leads to a red shift.