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AP Physics 1 Exam Review - Beats

With the release of our new AP Physics 1 content, we decided that it'd probably be best to cover a question from the sample question list we have available. In an earlier post, we discussed the major changes that AP Physics has gone through by being split into two separate subjects. With these new changes, it'll be important to see what type of questions will be on the exam and how they will be different from what was presented in AP Physics B. Something different the new exams include are questions that have two answers. Five of the questions on the exam will incorporate these multiple selection question types. Let's look at an example of a question you'd may see.

Beats are interferences caused by two sound waves. Which of the following statements applies to how beats are formed? There are two answers.

You answer choices are:

  • The beats are caused by constructive and destructive interference.
  • The beats are caused solely by constructive interference.
  • The two waves are completely in phase.
  • The two waves are similar in frequency, but slightly off.

The key to answering this question is understanding what beats are. Beats are a new concept covered in the AP Physics 1 course guideline that relate to waves. Beats are created when two waves with similar frequencies interfere with one another, but are slightly off phase. Waves that interfere with one another have two types of interference that can occur: constructive and destructive interference. Constructive interference (CI) occurs when two waves are in phase with each other, resulting in a superinposed wave that has an amplitude of magnitude equal to the sum of the two amplitudes of the individual waves.

Oppositely, when waves destructively interfere (DI), it means they are completely off phase, where the crest of one wave meets the trough of the other wave. This results in the waves cancelling each other out. Look at the image below which shows two slight off sync waves interfering with one another:

constructive and destructive interference

The above image is what occurs in a beat. The red and blue waves are constantly in sections of constructive and destructive interference. The areas of constructive intereference are when the added displacement of both waves results in a larger amplitude, as indicated by the orange wave. However, notice that the magnitudes start small and eventually increase until reaching a peak, and then begin to decrease again. This section of constant constructive interference before destructive interference occurs is specified as 1 beat. After destructive interference occurs, the waves begin to constructively interfere again as shown by the second darker blue section of the beat when the amplitude of the combined wave is greater than the amplitude of each single wave. The constant constructive and destructive interference occurring between the two waves could not be possible if it weren't for the two waves having similar frequencies.

If that long explanation didn't give you the answer, then the two answers to this question are 1.) The beats are caused by constructive and destructive interference and 2.) The two waves are similar in frequency, but slightly off.

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