RE: Wrong behavior of CrossProduct
- To: mathgroup at smc.vnet.net
- Subject: [mg8029] RE: [mg7996] Wrong behavior of CrossProduct
- From: "Richard W. Finley, M. D." <trfin at umsmed.edu>
- Date: Sat, 2 Aug 1997 22:32:38 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Sean,
Regarding the message below, perhaps I missed something....as far as I
know a vector with zero magnitude is the zero vector, regardless of
whether you consider it a displacement vector or a field vector, and the
cross product of any vector with this zero vector should be zero. This
would seem to be the only interpretation that makes mathematical OR
physical sense.
RF
-----Original Message-----
From: seanross at worldnet.att.net [SMTP:seanross at worldnet.att.net]
Subject: [mg7996] Re: [mg7958] Wrong behavior of CrossProduct
Sean Ross wrote:
... Looking at the problem you
gave mathematica in spherical coordinates you specified V3D(a1,a2,0},
which is a displacement vector beginning at the origin and going to the
point V. You then wanted to cross it with the vector U3D{0,0,1}, which
is a displacement vector beginning at the origin and ending at the
origin, so you took a cross product between two vectors, one of which
had a zero magnitude. The answer given by mathematica was correct for
DISPLACEMENT VECTORS. This makes perfect mathematical sense, but is
ludicrous from a physical standpoint since all cross-products that
appear in physical equations are for field vectors, not displacements.