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Gravitational elongation formula conversion from SI

  1. Apr 25, 2012 #1
    I have attached a pic which really explains the formula and context of the problem.

    I'm guessing a gravitational effect passes for physics....

    I am trying to convert the formula in (b) below into feet.

    If I could reproduce the answer with English units, I could move on and apply the formula to other situations, but the multiplicity of English units is really confusing me.
    The English units I have are;
    g = 32.17405
    x = 3277.972
    Next two from my tapes manufacturer,
    A = 0.003" (but I think it should be 0.034 feet)
    E = 29,700 KSI
    M = 0 but could be 20 lbs.
    Next one again from the manufacturer,
    m = 0.0102 lbs/ft.
    l = 3280.84
    P(sub)0 = ? but my hundred foot tape would be 23 lbs.

    Attached Files:

  2. jcsd
  3. Apr 27, 2012 #2


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    Science Advisor
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    A is the cross-sectional area. 0.003 sq in seems reasonable.
    The formula does not need to be converted to use it in different units.
    All you have to do is keep track of the units as though they were algebraic unknowns. If you've been completely consistent in choice of units they'll all cancel out. Otherwise you'll have terms like metres/foot remaining, and then you just substitute the appropriate constant (0.3 in this example).
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