Vector Operations
Multiplication and Division of a Vector by a Scalar. If a vector is multiplied by a positive scalar, its magnitude is increased by that amount. Multiplying by a negative scalar will also change the directional sense of the vector. Graphic examples of these operations are shown in Fig 2-2.
Vector Addition. When adding two vector together it is important to account for both their magnitudes and their directions. To do this we must use the parallelogram law addition. To illustrate, the two component vector A and B in Fig 2.3a are added to form a resultant vector R=A+B using the following procedure:
- First join the tails of the components at a point to make them concurrent,Fig. 2-3b.
- From the head of B, draw a line parallel to A. Draw another line from the head of A that is parallel to B. These two lines intersect at point P to form the adjacent sides of a parallelogram.
- The diagonal of this parallelogram that extends to P forms R, which then represents the resultant vector R=A+B,Fib.2-3c.
We can also add B to A,Fig.2-4a, using the triangle rule, which is a special case of the parallelogram law, where by vector B is added to vector A in a "head-to-tail" fashion, i.e., by connecting the head of A to the tail of B,Fig.2-4b . The resultant R extends from the tail of A to the head of B. In a similar manner, R can also be obtained by adding A to B,Fig.2-4c. By comparaison, it is seen that vector addition is commutative; in other words, the vector can be added in either order, i.e., R=A+b=B+a.
As a special case, if the two vector A and B are collinear, i.e., both have the same line of action, the parallelogram law reduces to an algebraic or scalar addition R=A+B, as shown in Fig 2-5.
Vector Subtraction. The resultant of the difference between two vectors A and B of same type may be expressed as
R = A - B = a + (-B)
This vector sum is shown graphically in Fig. 2-6. Subtraction is therefore defined as a special case of addition, so the rules of vector addition also apply to vector subtraction.