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Homework Help: Determine the number of N of digital bits

  1. Aug 26, 2011 #1
    On an audio compact disc (CD), digital bits of information are encoded sequentially along a spiral path. Each bit occupies about 0.28 um. A CD player's readout laser scans along the spiral's sequence of bits at a constant speed of about 1.2 m/s as the CD spins.

    Determine the number N of digital bits that a CD player reads every second.

    How should I start on this problem. Thank you !
  2. jcsd
  3. Aug 26, 2011 #2


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    If you had a conveyor belt going past at 5 m/s, and it was loaded with pencils arranged sideways on the belt, touching each other, and each pencil is 4mm in diameter - how many pencils pass each second.

    You CD is like that with different sizes involved.
  4. Aug 26, 2011 #3


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    Just FYI, the phrase "digital bits" is a nonsensical. "Bits" means BINARY digits, not decimal digits.
  5. Aug 26, 2011 #4
    So I would need to convert 5m/s to mm, which will then give me 5000mm/s?
    Then in order to fill the number of pencils pass each second, I would take 5000 mm/4 mm, which will then give me 1250 pencils?

    As for the problem I posted, um is micrometer, right?
    So then I would need to convert 1.2 m/s to 1200000 um/s?

    Then as I did before, I would need to take 1200000 um / 0.28 um, which will then give me 42.86E^6?
  6. Aug 26, 2011 #5


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    That should be correct [I didn't check the calculation]. though that answer would be marked wrong in our school system. You have merely written down what the calculator said.

    You should have said 4.3 x 10^7 [assuming you answer was correct] [or 43 x 10^6 if you are using engineering notation]

    As I said, any submitted answer here, with an E in it, marked WRONG.

    [and why only 4.3 rather than 4.286?]
  7. Aug 27, 2011 #6
    I calculated that, and entered it in the system as 4.3 x 10^7, and it was right.
    This was for an online homework problem.

    Thank you so much for your help.
    Have a great day !
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