AP Physics 2: 6.1 Waves – Exam Style questions with Answer- MCQ

Question

If the tension in a taut string is increased, which of the following will also be increased when the fundamental frequency is struck? Select two answers.
       (A) The velocity of propagation
       (B) The frequency of the fundamental
       (C) The wavelength of the fundamental
       (D) The amplitude of the wave

▶️Answer/Explanation

Ans: (A) and (B) Higher tension means waves will propagate at higher speeds. Since the wavelength of the standing wave is fixed by the length of the string, the frequency of the fundamental must also increase.

Question

 Two tuning forks are struck at the same time. A beat frequency of 12 beats per second  is observed. If one tuning fork has a frequency of 384 Hz, what could be the frequency of the second tuning fork? Select two answers.
        (A) 260 Hz
        (B) 372 Hz
        (C) 396 Hz
        (D) 408 Hz

▶️Answer/Explanation

Ans: (B) and (C) The beat frequency is caused by the difference between the two frequencies: 384 ± 12 = 396 or 372.

Question

 In the fixed standing wave shown below (imagine a string of fixed length), what will happen to the wavelength and frequency if the wave speed is raised while  the standing wave pictured remains unchanged?

(A) Wavelength increases while frequency remains the same.
(B) Wavelength remains the same while frequency increases.
(C) Both wavelength and frequency increase.
(D) Although we know the product of wavelength and frequency increases, we do not know what combination is producing this effect.

▶️Answer/Explanation

Ans: (B) The wavelength is set by the length of the standing wave and the number of the harmonic. Therefore the wavelength remains the same. The frequency of the wave must have been raised to correspond to the higher wave speed: λf = v.

Question

 Bats can find objects in the dark by using echolocation (sending out a high-frequency sound and listening to the echo). They also listen for a change in the pitch of the echo to
       (A) confirm the estimated distance of the object
       (B) determine the velocity of the object
       (C) determine whether the object is moving toward or away from them
       (D) estimate the approximate composition of the object

▶️Answer/Explanation

Ans:(C) The Doppler effect can be used to determine relative speed toward or away from the source by looking at frequency shift in the reflected wave. Note that the velocity vector cannot be determined. Components of velocity not directed toward or away from the receiver do not contribute                                              to the Doppler shift.

Question

 A 340-hertz tuning fork sets an air column vibrating in fundamental resonance, as shown above. A hollow tube is inserted into a column of water,                                       and the height of the tube is adjusted until strong resonance is heard. At approximately what length of th air column will this happen?
       (A) 100 cm
       (B) 75 cm
       (C) 50 cm
       (D) 25 cm

▶️Answer/Explanation

Ans: (D) The fundamental frequency in a half-pipe is ¼ wavelength:
                                    Wavelength = wave speed/frequency = 340/340 = 1 m
         The pipe needs to be ¼ of this: 0.25 m = 25 cm.

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