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Homework Help: Units of Conversion (Pulse)

  1. Aug 24, 2010 #1
    Time standards are now based on atomic clocks. A promising second standard is based on pulsars, which are rotating neutron stars (highly compact stars consisting only of neutrons). Some rotate at a rate that is highly stable, sending out a radio beacon that sweeps briefly across Earth once with each rotation, like a lighthouse beacon. Suppose a pulsar rotates once every 1.434 806 448 872 75 4 ms, where the trailing 4 indicates the uncertainty in the last decimal place (it does not mean 4 ms).

    (c) What is the associated uncertainty of this time?

    I attempted to solve the problem find the average between 1.434 806 448 872 79 and 1.434 806 448 872 71 (in seconds, meaning i already divided by 1000m/s) yet I could not get the correct answer.
    I also used the answer in part (b) as a base to find the average
    That also failed.
    I've got the correct answer for (b) but cannot figure out (c)

    Heres (b) in case you were interested
    b) How much time does the pulsar take to rotate 4.0 106 times? (Give your answer to at least 4 decimal places.)

    answer is: 5739.225795

    help please!!
  2. jcsd
  3. Aug 24, 2010 #2


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    Part (c) seems to ask for an associated uncertainty. So what is the definition of associated uncertainty? (It's not an average)
  4. Aug 24, 2010 #3
    well i attempted 1.43480644887275/00000000000004
    something like that

    the problem here i guess is that i don't understand associated uncertainty.
    any idea how to solve it/definition is?

    please and thx
  5. Aug 25, 2010 #4


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    Well, I'm actually not quite sure what it means either. The terms I usually hear are "relative uncertainty" or just "uncertainty", or occasionally "absolute uncertainty".

    Maybe the question means "What is the uncertainty associated with this time?" In that case they're just asking for the uncertainty, written out as a time (in milliseconds).
  6. Aug 25, 2010 #5
    Figured out the answer.
    Its actually very simple

    Multiply 4.0x10^6 from (b)
    by the uncertainty
    (which is .000 000 000 000 04).
    That gives you the answer in ms

    My final answer required it to be in seconds
    so I divided the ans by 1000 m/s
  7. Aug 25, 2010 #6


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    Ahhh, I see, they were asking for the uncertainty in the answer to part (b). I got thrown off a bit there because you wrote part (c) right after the introductory information.

    Anyway, glad you figured it out.
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