Name

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      INERTIATENS

Purpose

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      Calculates the 2nd moment tensor (sometimes incorrectly referred to
      as the moment of inertia tensor) of a mass distribution specified by
      a list of particle positions.

Category

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      Astro

Calling Sequence

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      Result = INERTIATENS(Pos)

Inputs

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      Pos: An Nx3 array specifying the 3d positions of the N particles
            that make up the mass distribution.

Keyword Parameters

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      MASSES: An N-element vector of the mass of each point. If not
              specified, all masses are assumed to be unity.
      R2WEIGHT: If /R2WEIGHT is specified then particles are downweighted
                by a factor of 1/r^2 so that all particles have equal
                effect regardless of radius.

Outputs

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      The function returns a 3x3 symmetric array containing the 2nd moment
      of the mass distribution tensor, i.e. Result[i,j] is the sum over each
      particle k of MASSES[k] * Pos[k,i] * Pos[k,j].

Example

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      Calculate the inertia tensor of 6 equal-mass points distributed on
      the vertices of a cube:
      xmasspos = [-1,-1,-1,-1,1,1,1,1]
      ymasspos = [-1,-1,1,1,-1,-1,1,1]
      zmasspos = [-1,1,-1,1,-1,1,-1,1]
      itens = INERTIATENS([[xmasspos],[ymasspos],[zmasspos]])

Modification History

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      Written by: Jeremy Bailin
      10 June 2008 Public release in JBIU
      22 July 2011 Bug fix for /R2WEIGHT to actually do something useful