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Homogeneous but incorrect?

  1. Nov 9, 2006 #1
    im posed with the question why an equation may be homogeneous with respect to its units but still be incorrect? i cant think of way to explain this can anyone help me out? Thanx
     
  2. jcsd
  3. Nov 9, 2006 #2

    vanesch

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    Consider a rod with length L1 = 15 cm and another rod with length L2 = 10cm.

    Now put the two rods one after the other.
    Consider the following formula: the total length of the two rods L = L1 - L2 = 5 cm.

    This formula is dimensionally correct, and the answer is wrong.

    Another formula:

    the total length equals L = L1 L2 / (L1 + L2) = 150 cm^2 / 25 cm = 6 cm

    Again, the formula is dimensionally correct, but gives the wrong anwer.

    ...
     
  4. Nov 9, 2006 #3
    Thanx, thts a good way to describe it. Is there any well known common formulae that fits this trend?
     
  5. Nov 9, 2006 #4

    Office_Shredder

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    Force is ma = kgm/s2

    So imagine we have an object with mass m and length L just sitting there for some period of time t. Then the net force F=m*L/t2 doesn't actually turn out to be 0.
     
  6. Nov 9, 2006 #5

    russ_watters

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    A well-known formula that doesn't work? No... :confused:
     
  7. Nov 9, 2006 #6

    vanesch

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    Pick a pile of exam copies of first year students :biggrin:
     
  8. Nov 10, 2006 #7

    HallsofIvy

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    Except that it is probable that none of those will be dimensionally correct either!
     
  9. Nov 10, 2006 #8

    vanesch

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    :rofl: :rofl:
    Right !
     
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