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Inertia and Wr^2 GD^2 and mr^2

  1. Mar 1, 2005 #1
    trying to figure out units for inertia.

    am I right in saying some units for inertia are?:

    imperial units - not interested (Wr^2)
    kgfm^2 (GD^2)
    kgm^2 (mr^2)

    if so, what is the f and the G and the D?

    Thanks for any help.
     
  2. jcsd
  3. Mar 1, 2005 #2

    Andrew Mason

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    The units are that of mass: kg, g, lb, oz. Mass is a measure of the inertia of an object - the ratio of force to acceleration. Inertia = f/a = m

    AM
     
  4. Mar 1, 2005 #3
    sorry, I meant moments of intertia. ie for a rotating body. It's just really the f that is putting me off.
     
  5. Mar 1, 2005 #4

    dextercioby

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    That's an ancient (b4 1961) unit for force.It's Kgf=Kilogram-force...The gravitational acceleration (average) at the surface of the Earth times 1Kg.That explains G...As for D,i guess it's probably distance (?)...

    The last of the 3 units presented is the correct (SI-mKgs) one.

    Daniel.
     
  6. Mar 1, 2005 #5

    Andrew Mason

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    Ok. Just use [tex]I = \tau/\alpha[/tex].

    Since torque is in units of force * distance or Nm, and [itex]\alpha[/itex] is [itex]rad/sec^{2}[/itex], the units are [itex]Nmsec^2[/itex] (or [itex]kgm^2[/itex] since [itex] N = kgm/sec^2[/itex]). As dexter says, kgf is kilograms force which is 1 kg x 9.8m/sec^2.

    AM
     
  7. Mar 1, 2005 #6
    thanks guys, I had a hunch that was what the f was. And the D is diameter.

    knarl.
     
  8. Mar 1, 2005 #7

    dextercioby

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    Weight times diameter doesn't make any sense...Even elimination would lead you to the correct answer.

    Daniel.
     
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