On 11/19/2003 12:58 PM, Joseph Bellina wrote:
> One think I do is make a clear distinction between "speed" and
> "velocity",
Good!
> so I wouldn't talk about negative speed,
Yes, certainly there is no such thing as negative
speed. Speed is |v|, which is never going to be
negative.
> only negative velocity.
Lost me there.
Except in D=1, it is a misnomer to speak of negative
velocity. To my ears, "negative" means "less than zero"
and there is no "less-than" operator that makes
sense in D=2 or greater.
(And even in D=1 it's a bad habit to speak of negative
velocity.)
You can say certain components of the velocity
(relative to a chosen basis) are negative.
Similarly you can say that one velocity vector is the
negative of another, i.e. equal and opposite ...
but this is not at all equivalent to saying the
first vector "is negative" i.e. less than zero.
More generally you can say that one vector has a
negative projection in the direction of another.