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The Graphical Method for Vector Addition and Scalar Multiplication

TopicsThe Graphical Method for Vector Addition and Scalar Multiplication

Graphical Addition

Consider the vectors u = (3, 4) and v = (4, 1) in the plane. From the
component method of vector
addition we know that the
sum of these two vectors is u + v = (7, 5). Graphically, we see that this is the
same as the result we would get by "picking up" one of the vectors (without
changing either its direction or its magnitude), placing its end at the
other (unmoved) vector's tip, and drawing an arrow from the origin to the new
tip location for the displaced vector.

This geometric procedure for adding vectors works in general. For any two
vectors u and v in the plane, the sum of the vectors is graphically given as
in the following figure:

The geometric procedure is valid for 3-dimensional vectors as well.
Notice that in the same way that any two lines lie in a plane, any two
vectors in 3-dimensional space will also lie in the same plane. This
recognition allows us to see that the sum of two vectors will always lie in the
plane defined by the original two vectors.

As we noted in Vector
Subtraction, in order to
subtract one vector from another, you simply add its negative partner:
u - v=u + (- 1)v. Thus, vectors can be subtracted graphically in the same manner
used for adding them, by simply taking care to reverse the direction of the
vector being subtracted:

If you graphically add back in the subtracted vector to your result from the
subtraction and you recover the initial vector you subtracted from. In other
words, (u - v) + v = u in our graphical methods, as we should expect!

Scalar Multiplication

What happens graphically when we multiply a vector by a scalar? The vector
changes in length, while its direction remains the same. If the vector's
magnitude was previously | v|, once it is multiplied by a scalar we have
| av| = a| v|. Note that if | a| > 1 the new vector will be longer. If
| a| < 1 the new vector will be shorter. And if a < 0, the new vector will point
in the opposite direction as the original one.