Dot Product and Angle

Given two vectors and , let be the angle between them such that . We will refer to as the included angle.

The following theorem establishes a relationship between the dot product and the included angle.

Proof
Consider the triangle formed by , and .

By the Law of Cosines we have: \begin{align*} \norm{\vec{u}-\vec{v}}^2=\norm{\vec{u}}^2+\norm{\vec{v}}^2-2\norm{\vec{u}}\norm{\vec{v}}\cos \theta \end{align*}

By Theorem th:dotproductpropertiesitem:norm of Dot Product and its Properties \begin{align*} (\vec{u}-\vec{v})\dotp (\vec{u}-\vec{v})=&\vec{u}\dotp \vec{u}+\vec{v}\dotp \vec{v}-2\norm{\vec{u}}\norm{\vec{v}}\cos \theta \end{align*}

By Theorem th:dotproductpropertiesitem:distributive-again of Dot Product and its Properties \begin{align*} (\vec{u}-\vec{v})\dotp \vec{u}-(\vec{u}-\vec{v})\dotp \vec{v}=&\vec{u}\dotp \vec{u}+\vec{v}\dotp \vec{v}-2\norm{\vec{u}}\norm{\vec{v}}\cos \theta \end{align*}

By Theorem th:dotproductpropertiesitem:distributive of Dot Product and its Properties \begin{align*} \vec{u}\dotp \vec{u}-\vec{v}\dotp \vec{u}-\vec{u}\dotp \vec{v}+\vec{v}\dotp \vec{v}=&\vec{u}\dotp \vec{u}+\vec{v}\dotp \vec{v}-2\norm{\vec{u}}\norm{\vec{v}}\cos \theta \end{align*}

By Theorem th:dotproductpropertiesitem:commutative of Dot Product and its Properties \begin{align*} -2(\vec{u}\dotp \vec{v})=&-2\norm{\vec{u}}\norm{\vec{v}}\cos \theta \\ \vec{u}\dotp \vec{v}=&\norm{\vec{u}}\norm{\vec{v}}\cos \theta \end{align*}

Orthogonal Vectors

We can use Theorem th:dotproductcosine to show that two non-zero orthogonal vectors of are simply perpendicular vectors (the included angle is ). To see this, suppose that for nonzero vectors . Then from Theorem th:dotproductcosine we have Since are nonzero vectors, we have , which implies . The converse also holds. If , then the dot product is clearly 0.

The reason we prefer the term “orthogonal” to “perpendicular” in this course is because is only one example of a vector space, and the dot product is only one example of a more general product, called an inner product. For vectors in a zero dot product happens to coincide with the geometric idea of perpendicularity, but there are many vector spaces that do not possess the visual geometry of . (Later in the text, you will encounter vector spaces whose vectors are polynomial functions!) In these more abstract settings, a zero inner product still signals a special relationship between vectors. The term orthogonal captures this relationship.

Practice Problems

Problems prob:anglebetweenvectors1-prob:anglebetweenvectors4

Find the degree measure of the included angle, for each pair of vectors. Round your answers to the nearest tenth.

and .

Answer:

and

Answer:

and

Answer:

and

Answer:

What does the sign of the dot product tell us about the included angle?
Find all values of so that is orthogonal to . List your answers in increasing order.

Answer: .

Find the value of for which the vector is parallel to the vector . What is the measure of the included angle, ? Find the measure of the included angle using Theorem ex:anglebetweenvectors. Do the two results agree?

Answer:

Prove that if is a unit vector, then .
Prove that if and are unit vectors, then . In what cases are the extreme values of 1 and attained?
Imagine a clock with hands represented by vectors and , as shown below. At what whole hour will attain its maximum value? At what whole hour will be as small as possible?

PIC

Answer:

Let be a circle of radius . Suppose and are the endpoints of a diameter of , and is a point on distinct from and . Show that vectors and are orthogonal.
Assign coordinates to points , and , express vectors and in component form, then find the dot product of and .

A rhombus is a quadrilateral with four congruent sides. Use vectors to prove that a parallelogram is a rhombus if and only if its diagonals are perpendicular.
See section on vector subtraction in Vector Arithmetic.
The points , and form a triangle in . Is it a right triangle?
Express each side of the triangle as a vector and use what you have learned in this section.

Photo Credits

The following images are courtesy of Wikimedia Commons

Hannes Grobe, Wall clock manufactured by Telefonbau & Normalzeit. CC-BY 3.0

2024-09-11 17:58:51