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Dimensionless Quantities

  1. Apr 17, 2012 #1
    Why are pure numbers like 1 , 2 , 3 .... dimensionless and Avogadro's Number, Plank's Constant, Gravitational Constant dimensional ?
  2. jcsd
  3. Apr 17, 2012 #2


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    Because the latter have dimensions! The Gravitational constant, G, appears in F= -GmM/r^2. In the metric system (mks), r has units of meters, both m and M have units of kg so that "mM/r^2" have units of [itex]kg^2/m^2[/itex]. F, a force, has units of "[itex]kg m^2/sec^2[/itex]". In order to make the units on both sides of the equation the same, G must have units of [itex]1/(kg sec^2)[/itex].

    Avogadro's number is the number of molecules per mole. The number of molecules does not depend on any units so Avogadro's number has units of [itex]mol^{-1}[/itex].

    Plank's constant is the "h" in [itex]h\nu[/itex] where E is energy, and so has units of [itex]kg m^2/sec^2[/itex] while [itex]\nu[/itex], a frequency, is "number of cycles per second". "Number of cycles", like "number of molecules" is just a number without units. Since we need to have left "[itex]kg m^2/sec^2[/itex]" we need . That means that h must have units of [itex]kg m^2[/itex] in the numerator and one "sec" in the denominator: [itex]kg m^2/sec[/itex].
  4. Apr 17, 2012 #3
    Why is 1/2 in 1/2 mv^2 dimensionless ?
  5. Apr 17, 2012 #4
    Because mv^2 has the unit same as that of Energy.
  6. Apr 17, 2012 #5
    Please elaborate it mathematically.
  7. Apr 17, 2012 #6
    Let this constant (which turns out to be 1/2) be C. Let k and k' be different dimensionless numbers. It's quite easy to see that we can set up

    [tex]C\cdot k\cdot \left(1\ \mathrm{J}\right)=k'\cdot\left(1\ \mathrm{J}\right)[/tex]

    And so


    And so C's dimensionless.

    I wanted to point out that c, the speed of light, is dimensionless, equal to 1, in Special (and General, I'd imagine) Relativity. Basically, [itex]299792458\ \mathrm{m}=1\ \mathrm{s}[/itex].

    I've always found it useful to think of units as constants that are, in some cases, incompatible with one another, so the simplest form is just their product.
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