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Minus one means the opposite of whatever convention you choose to be
positive. For example If you say that all positions to the right side
of where you are standing is positive then all positions to the left of
where you are standing can be regarded as negative.
Example: Imagine we are looking at Joe, Jack and Jill FROM THE BACK. So
Jill is 3 meters to the right of Jack and Joe is 6 meters to the left of
Jack. In this case the minus sign meant "to the left of Jack". Relative
positions can always be found be subtraction then.
Joe Jack Jill
0 0 0
-O- -O- -< >-
O O /_\
_________W________________W_________W_________
| | |
-6m 0 3m
The position P of Jill relative to Joe is
P(Jill rel. to Joe) = 3m - (-6m) = 9m
The plus sign tells us that Jill is 9m TO THE RIGHT of Joe.
The position of Joe relative to Jill is
P(Joe rel. to Jill) = -6m - (3m) = -9m
The minus sign tells us that Joe is 9m TO THE LEFT of Jill.
This is an example of how using a minus sign convention in physics allows
us to describe not only how far but also in what direction. There are
many other uses for negative numbers in physics to describe phenomena that
oppose other phenomena. There is even meaning to complex numbers (!) ie
sqrt(-1). Perhaps someone will write us and ask about that someday.
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