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Physics 102

15 Minute Quiz

Sept 30, 2003
Fill in the circle corresponding to your answer (example:   e.g. l phase). Each question counts 10 points.

  1. We have an audible wave that remains constant in pitch but becomes softer. What feature of the sound wave changes?
    1. ¡ frequency
    2. l amplitude
    3. ¡ wave shape
    4. ¡ period
    5. ¡ phase
    expect to find a lot of these kind of questions on the hour exams -- with at most 5 possible answers. All the usual "tricks" for multiple-choice tests apply: it pays to take your best guess, there is no penalty for wrong answers. You can't get less than zero!. If two answers say essentially the same thing -- like "frequency" and "period" above -- neither of them is likely to be the answer, because the answer must be unique, or at least the "best answer".
  2. In a lecture demo two speakers were placed close together and connected to the same music source. For one way of connecting them the low, bass notes were prominent. When a switch was thrown (reversing the wires connected to one of the speakers), the bass could hardly be heard. What happened?
    1. l After reversal the speakers' sound waves were out of phase and canceled
    2. ¡ Loudspeakers work only when connected correctly, so the speaker with the reversed wires put out no sound
    3. ¡ One way of connecting a speaker emphasizes the bass, reversing the wires emphasizes the treble
    4. ¡ The electrical signals fed into the speakers interfered with each other
    5. ¡ The reversal was a fake, as shown when the speakers were pulled apart and then worked perfectly well
    We do consider lecture demos an important part of your physics experience. The syllabus tells you how to find out about all the demos that were planned to be shown on a given date, and the "more" sections of the schedule outline mention the more important demos. Even if you have not seen the demo, you can eliminate patently absurd answers -- we have to resort to those because it's not so easy to find clearly wrong answers!
  3. Two sine waves of amplitude A are fed into a scope to display a Lissajous figure. This figure is a 45° straight line from upper left to lower right (). In a second experiment the two waves are added together. The result is a wave of amplitude
    1. l 0 (zero)
    2. ¡ ½A
    3. ¡ A
    4. ¡ Ö2 A
    5. ¡ 2A
    Sometimes there is a question where you have to "think" -- that is, put two or more things together. Generally the question will not give you more information than you need. If you have not used all the info of the question, something may be wrong. In this case the result of the first experiment is essential to tell you that the waves are out of phase
  4. A loudspeaker emits sound of frequency 1 kHz = 1000 Hz into air where the speed of sound is 345 m/s. The wavelength l of the sound wave is
    1. ¡ 345,000 m
    2. ¡ 345 m
    3. ¡ 3.45 m
    4. l 0.345 m
    5. ¡ 345 cm
    You will probably rely on a calculator, but the numerical questions are generally designed so you can do them in your head -- at least if your head is properly prepared mathematically. You can check yourself in various ways; one is to have faith that the problem was posed so the answer is reasonable. In this case all you have to know is generally what the wavelength of sound is, to eliminate the two largest numbers. Two others are the same number expressed in m and in cm, respectively -- that leaves only one!
  5. The same loudspeaker with its 1 kHz frequency now emits the sound into Helium, where the sound speed is three times (3 ×) that in air. The wavelength in Helium, compared to that in air, is
    1. l 3 × larger
    2. ¡ the same
    3. ¡ 1/3 × that in air
    4. ¡ 1/9 × that in air
    5. ¡ not enough information is given to compute it
    Answers like "not enough information", "none of the above" etc. may indicate that I could not think of another plausible answer. But be careful of "all of the above". In that case, don't pick an answer you know is right without checking whether all the others are also right!
  6. An echo is an example of     (which change?)     of sound:
    1. ¡ refraction
    2. ¡ diffraction
    3. ¡ detraction
    4. l reflection
    5. ¡ deflection
    This type of question checks not physics but vocabulary. It occurs occasionally since you should know some of the lingo as well as the concepts of the subject. You'll often find similar-sounding absurdities (like detraction) among the choices. And apologies to any foreign friends who use a different spelling (like "reflexion"). Be suspicious of, but do not reject out of hand, anything that seems spelled wrong.
  7. The inverse square law tells exactly how the     (which physical feature?)     of a sound wave decreases as the distance between source and observer increases:
    1. ¡ amplitude
    2. l intensity
    3. ¡ loudness
    4. ¡ damping
    5. ¡ inverse square
    This one almost goes against the advice of problem 1: Amplitude, intensity and loudness may seem to be the same thing, hence to be eliminated. 3 answers that can be eliminated would however show a deplorable lack of imagination on the part of the test designer, so be suspicious. And yes, it helps to have been in lecture and notice that the difference between the three seems to be a pet point with the lecturer, that he mentioned several times ...
    Finally, as a rewward for having gotten this far, a small hint: though some exam designers avoid it, there is nothing ipso facto wrong, on Exam 1, if you find yourself answering with the same letter several times in a row!
  8. On the axes below question 10, draw exactly two periods of a square wave of amplitude 1 V and period 1 ms.

  9. On the same figure, draw a triangle wave with the same frequency and amplitude, and in phase with the square wave (that is, crossing zero at the same times).

  10. On the same figure, draw the result of adding the two waves. Clearly label it "sum".