Vector Algebra
Figure A.2:
Vector addition.
|
Suppose that the displacements
and
represent the vectors
and
, respectively. See Figure A.2. It can be seen that the result
of combining these two displacements is to give the net displacement
. Hence,
if
represents the vector
then we can write
![$\displaystyle {\bf c} = {\bf a} + {\bf b}.$](img4489.png) |
(1.1) |
This defines vector addition.
By completing the parallelogram
, we can also see that
![$\displaystyle \stackrel{\displaystyle \rightarrow}{PR} \,= \, \stackrel{\displa...
...ackrel{\displaystyle \rightarrow}{PS}+\stackrel{\displaystyle \rightarrow}{SR}.$](img4491.png) |
(1.2) |
However,
has the same length and direction as
,
and, thus, represents the same vector,
. Likewise,
and
both represent the vector
. Thus, the previous equation is equivalent to
![$\displaystyle {\bf c} = {\bf a} + {\bf b} = {\bf b} + {\bf a}.$](img4494.png) |
(1.3) |
We conclude that the addition of vectors is commutative. It can also
be shown that the associative law holds; that is,
![$\displaystyle {\bf a} +
({\bf b} + {\bf c}) = ({\bf a} + {\bf b}) + {\bf c}.$](img4495.png) |
(1.4) |
The null vector,
, is represented by a displacement of zero length and arbitrary direction.
Because the result of combining such a displacement with a finite length displacement is the same
as the latter displacement by itself, it follows that
![$\displaystyle {\bf a} + {\bf0} = {\bf a},$](img4496.png) |
(1.5) |
where
is a general vector.
The negative of
is defined as that vector that has the same magnitude, but acts in the opposite direction, and is denoted
.
The sum of
and
is thus
the null vector; in other words,
![$\displaystyle {\bf a} + (-{\bf a}) = {\bf0}.$](img4498.png) |
(1.6) |
We can also define the difference of two vectors,
and
, as
![$\displaystyle {\bf c} = {\bf a} - {\bf b} = {\bf a} +(-{\bf b}).$](img4499.png) |
(1.7) |
This definition of vector subtraction is illustrated in Figure A.3.
Figure A.3:
Vector subtraction.
|
If
is a scalar then the expression
denotes a vector whose direction is the same
as
, and whose magnitude
is
times that of
. (This definition becomes obvious when
is an integer.)
If
is negative then, because
, it follows
that
is a vector whose magnitude is
times that of
, and whose
direction is opposite to
. These definitions imply that if
and
are
two scalars then