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physics practice exam solution

# physics practice exam solution - Prep101...

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© Prep101 http://www.prep101.com/freestuff Page 1 of 16 PRACTICE PHYSICS 101 EXAM Problem 1 I. A student, holding a 500 Hz tuning fork, is walking at 1.50 m/s on a direct line between two parallel smooth reflecting walls. What frequency of sound are heard by the student? Answer At the wall the student approaches, = s v v v f f ' because the wall is a stationary “observer” watching a moving source. For the sound reflected by the wall, the wall is a stationary “source” that reflects the same frequency it “hears”, and the student is a moving observer, so if the student hears frequency f’’, where: Hz s m s m s m s m Hz v v v v f v v v f f s o o 4 . 504 / 5 . 1 / 341 / 5 . 1 / 341 500 ' ' ' = + = + = + = Similarly, for receding wall, (the student is going away from the wall, the velocities have inverse signs): Hz s m s m s m s m Hz v v v v f f s o 6 . 495 / 5 . 1 / 341 / 5 . 1 / 341 500 ' ' = + = + = II. A column of air in a tube is found to have standing waves at frequencies of 380, 532 and 684 Hz. There are no standing wave frequencies between the above frequencies. a) What is the fundamental frequency of the tube? Answer Resonant frequency for sound waves in a tube are uniformly spaced. Notice the frequency spacing here is 152 Hz because 684Hz- 532Hz=532Hz-380Hz=152Hz. Keep subtracting multiples of 152Hz until you reach a frequency <152Hz and you’ll reach the fundamental frequency f fundamental =76Hz. b) Is the tube open at both ends or open at one end and closed at the other end? Answer For a tube open at both ends, the fundamental frequency is the same as the frequency of spacing. For a tube closed at one end, the fundamental frequency is on half of the frequency of spacing. The latter is true, so the tube is closed at one end.

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