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## The scalar product

A scalar quantity is invariant under all possible rotational transformations. The individual components of a vector are not scalars because they change under transformation. Can we form a scalar out of some combination of the components of one, or more, vectors? Suppose that we were to define the ampersand'' product,
 (16)

for general vectors and . Is invariant under transformation, as must be the case if it is a scalar number? Let us consider an example. Suppose that and . It is easily seen that . Let us now rotate the basis through about the -axis. In the new basis, and , giving . Clearly, is not invariant under rotational transformation, so the above definition is a bad one.

Consider, now, the dot product or scalar product:

 (17)

Let us rotate the basis though degrees about the -axis. According to Eqs. (10)-(12), in the new basis takes the form
 (18)

Thus, is invariant under rotation about the -axis. It can easily be shown that it is also invariant under rotation about the - and -axes. Clearly, is a true scalar, so the above definition is a good one. Incidentally, is the only simple combination of the components of two vectors which transforms like a scalar. It is easily shown that the dot product is commutative and distributive:
 (19)

The associative property is meaningless for the dot product, because we cannot have , since is scalar.

We have shown that the dot product is coordinate independent. But what is the physical significance of this? Consider the special case where . Clearly,

 (20)

if is the position vector of relative to the origin . So, the invariance of is equivalent to the invariance of the length, or magnitude, of vector under transformation. The length of vector is usually denoted (the modulus of '') or sometimes just , so
 (21)

Let us now investigate the general case. The length squared of (see Fig. 5) is
 (22)

However, according to the cosine rule'' of trigonometry,
 (23)

where denotes the length of side . It follows that
 (24)

Clearly, the invariance of under transformation is equivalent to the invariance of the angle subtended between the two vectors. Note that if then either , , or the vectors and are perpendicular. The angle subtended between two vectors can easily be obtained from the dot product:
 (25)

The work performed by a constant force moving an object through a displacement is the product of the magnitude of times the displacement in the direction of . If the angle subtended between and is then

 (26)

The rate of flow of liquid of constant velocity through a loop of vector area is the product of the magnitude of the area times the component of the velocity perpendicular to the loop. Thus,

 (27)

Next: The vector product Up: Vectors Previous: Vector areas
Richard Fitzpatrick 2006-02-02