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Formula for Density

  1. Mar 7, 2010 #1


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    I know that the formula for density is mass/volume. However, this is only true for rest mass. What about when the particle is moving?

  2. jcsd
  3. Mar 7, 2010 #2


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    Staff: Mentor

    This opens up the whole can of worms about the different kinds of mass in relativity. I suppose the answer would depend on what you want to use the density of the moving object for.
  4. Mar 7, 2010 #3


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    Mass increases with velocity, right?

    Do I have to use Einstein's General Theory of Relativity and somehow use that with my original density formula? I'm confused
  5. Mar 8, 2010 #4
    Note that the particle itself would not know if it is moving or not, unless it was being accelerated. So according to the particle, it's density would not increase.
    However, from the viewpoint of an external observer, it could be said that the density of the object had, in a way, increased.

    In the end, whether this density is what would fit in with your usual concept of density or not, depends on what you want to use it for, as jtbell said.
  6. Mar 17, 2010 #5


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    I don't get it... what do you mean by this?
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