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MathGroup Archive 1992

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Re: CrossProduct[ ]

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: CrossProduct[ ]
  • From: keiper
  • Date: Fri, 7 Aug 92 17:27:40 CDT
  • Cc: bugs, emily, swolf

There seems to be a misunderstanding about CrossProduct[ ] in
Calculus`VectorAnalysis`.  In particular, when the default coordinate
system is Cylindrical (or anything but Cartesian) this refers to
the global coordinate system.  Grad[ ] gives results in terms of the
local, infinitesimal coordinate system which is NOT Cylindrical,
but rather Cartesian.  Thus when you use CrossProduct[ ] (or DotPrduct[ ]
or ScalarTripleProduct[ ]) with vectors from the local, infinitesimal
coordinate system, you need to specify that you are refering to a
Cartesian coordinate system:

In[1]:= <<Calculus`VectorAnalysis`

In[2]:= SetCoordinates[Cylindrical]

Out[2]= Cylindrical[r, theta, z]

In[3]:= CrossProduct[Grad[r], Grad[theta], Cartesian]

               1
Out[3]= {0, 0, -}
               r

It could well be argued that this is a misfeature.  Does one ever
want to take the cross product of vectors in anything other than
a Cartesian system?  If so how often?  Should the default be
Cartesian with the ability to specify something else?  I can
easily change this, but it is not the sort of mathematics that I
use and I don't know what the default should be.

Jerry B. Keiper
keiper at wri.com




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