AP Physics 1 Question 138: Answer and Explanation

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Question: 138


The preceding diagram shows a speaker mounted on a cart that moves to the right at constant speed v. Wave fronts for the constant-frequency sound wave produced by the speaker are indicated schematically in the diagram. Which of the following could represent the wave fronts produced by the stationary speaker playing the same note?

  • A.
  • B.
  • C.
  • D.

Correct Answer: C


C-The Doppler effect states that a moving speaker will cause a stationary observer to hear a higher frequency. This is because the wave fronts will be emitted closer together than if the speaker were stationary-the waves will seem "scrunched" together. Well, for the wave fronts to be "scrunched" in the diagram for the moving cart, they'd have to be farther apart in the diagram for the stationary cart.