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# exam1v2ans - 0002-1 Exam I Physics 1240 Fall 2010 Version...

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Exam I – Physics 1240 – Fall 2010 Version 0002 1. Professor Betterton puts a strobe light in front of a spinning fan and adjusts the frequency of the strobe to the highest possible frequency so that what you see in class is the fan blade “frozen” in one position. The strobe is flashing with a frequency of 50 Hz. How many rotations per minute does the fan make? A) 25/min B) 600/min C) 1500/min D) 3000/min E) 9000/min 2. Suppose the strobe frequency is doubled (compared to the previous problem). What will you see when you look at the fan? A) It will look like it would with normal room illumination, you'll see the fan blade spinning as usual. B) Exactly the same as before, the fan blade is “frozen” in one position. C) The fan blade “frozen” in the upside-down (opposite) position compared to before. D) The fan blade rotates around very slowly. E) A kind of “double exposure” – you see the fan blade froze in the original position and in the upside-down (opposite) position compared to before. 3. A signal generator drives a speaker. Next to the speaker is a microphone, which is connected to an oscilloscope. The oscilloscope shows a pattern (see diagram on the right). The vertical axis measures Volts (which is what the microphone produces), the horizontal axis measures time in ms (milliseconds). What is the frequency of this sound wave? A) 0.05 Hz B) 20 Hz C) 25 Hz D) 50 Hz E) 100 Hz 4. If a tuning fork vibrates steadily at 1.5 kHz, what is the approximate physical distance in the room between one high pressure peak, and the next one, at any given instant in time? (Assume the temperature is 20 °C.) A) 344 m B) 23 m C) 4.4 m D) 23 cm E) 44 mm The next two questions address what happens when the tuning fork of the previous problem is moved to a much warmer room where the temperature is 40 °C. The tuning fork still vibrates steadily at 1.5 kHz. 5. What happens to the speed of the sound wave produced because of the increase in temperature? A) It goes down. B) It stays the same. C) It goes up. D) Not enough information to determine. 6. What happens to the wavelength of the sound produced because of the increase in temperature? A)

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exam1v2ans - 0002-1 Exam I Physics 1240 Fall 2010 Version...

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