MathGroup Archive 2001

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Engineer's Stunt

  • To: mathgroup at
  • Subject: [mg30674] Engineer's Stunt
  • From: bruce.detterich at (Bruce Detterich)
  • Date: Sat, 8 Sep 2001 02:22:14 -0400 (EDT)
  • References: <9msos5$plr$>
  • Sender: owner-wri-mathgroup at

The prior descriptions were a bit too complicated for my poor simple
mind.  Here's a little trick I use to remember how to generate Cross
product and the 3-dimensional special case, "Curl".

To generate the cross product of two vectors, e.g. A X B, assuming the
conventional unit vectors: i, j and k, simply expand by minors the
matrix whose first row elements are: i, j and k; second row elements
are: Ax, Ay, Az; and third row elements are: Bx, By, and Bz.

The Curl of B can be created by simply replacing the vector A in the
above example by the row vector of partial derivative operators: d/dx,
d/dy and d/dz.

I use the term 'expand my minors' to mean taking each element of the
first row, signed per usual convention, and multiplying it by the
result of expansion of its related 2X2 minor.

Again, don't forget the sign reversal of the j element due to its
'odd' sum-of-indices.
I assume that Mathematica uses some vastly more sophisticated approach
to the problem, but I know the above is pretty much bullet proof.

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