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Homework Help: Confusing Old time Measurements =(

  1. Jan 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A furlong is and old British unit of length equal to 0.125 mi, derived from the length of a furrow in an acre of plowed land. A fortnight is a unit of time corresponding to two weeks, or 14 days and nights. Find the speed of light in megafurlongs per fortnight. (one megafurlong equals a million furlongs).


    2. Relevant equations

    speed= distance/time
    fur=0,125 mi
    for=2 weeks
    mg= 1,000,000 fur

    3. The attempt at a solution

    1 mf/ 1,000,000 fur x 1 fur/0.125 mi x 0.125 mi/ 1fur x 2w/1 for = ... i'm stumped here. i cant understand which way to set it up so that i am only left with mf/for
     
  2. jcsd
  3. Jan 21, 2010 #2

    kuruman

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    If you are inexperienced with this, I suggest that you do it in steps instead of trying all at once. I will get you started, but you will have to finish.

    You start with what you know. The speed of light is c = 186,000 mile/hr. So you write

    [tex]c=186,000\frac{mile}{hr}[/tex]

    Now you are given that

    [tex]1 mile=\frac{1}{0.125}fu[/tex]

    Put that in the previous expression to get

    [tex]c=186,000\frac{1}{0.125}\frac{fu}{hr}[/tex]

    Now

    [tex]1fu=10^{-6} Mfu[/tex]

    Put that in the expression to get

    [tex]c=\frac{186,000\times10^{-6}}{0.125}\frac{Mfu}{hr}[/tex]

    Finish the question by proceeding along these lines to get the fortnight in the denominator. Incidentally, you don't really need the 186,000 there. You could as well start with 1 mile/hr to get the conversion factor then multiply the result by 186,000.


    *** Correction ***

    As pointed out by mgb_phys, the speed of light is 186,000 miles/sec. So the conversion is from "sec" to "fortnight". My apologies for the confusion.
     
    Last edited: Jan 21, 2010
  4. Jan 21, 2010 #3

    CompuChip

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    The speed of light in SI units is defined as (approximately) [itex]3 \times 10^8[/itex] m/s.
    How many miles per second is this?
    So how many furlongs per second?
     
  5. Jan 21, 2010 #4

    mgb_phys

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    First you need to estimate the answer so you know if you got it right.
    A furlong is less than a mile so a speed in furlongs/hour would be a larger number than mi/hour (ie at the same speed you do more furlongs than you do miles) - just like a speed in km/h is a bigger number than in mph.
    But a fortnigth is much bigger than an hour so at the same speed you would cover a much bigger distance. So the same speed would be a much bigger number in f/f

    Converting is just a matter of fractions.

    Assuming you start in miles/sec (c = 186,000 mi/s)
    furlongs / fortnight = miles/second * ( fortnight/second) / ( furlongs / mile )

    furlongs/mile = 0.125, fortnight/second = 14*24*3600
    So 186,000 * (14*24*3600/0.125) =
     
  6. Jan 21, 2010 #5

    mgb_phys

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    kuruman - slight typo, it's 186,000 mi/SECOND

    Teenytiny1991 - using Furlong/Fortnight/Firkin is to teach how to do these sort of conversion. It's just a bit more fun than doing mi/s to m/s or J to eV.
     
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