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Mathematics LibreTexts

10.9: Vectors

Skills to Develop

In this section you will:
  • View vectors geometrically.
  • Find magnitude and direction.
  • Perform vector addition and scalar multiplication.
  • Find the component form of a vector.
  • Find the unit vector in the direction of v.
  • Perform operations with vectors in terms of i and j.
  • Find the dot product of two vectors.

An airplane is flying at an airspeed of 200 miles per hour headed on a SE bearing of 140°. A north wind (from north to south) is blowing at 16.2 miles per hour, as shown in Figure 10.9.1. What are the ground speed and actual bearing of the plane?

Image of a plan flying SE at 140 degrees and the north wind blowing

Figure 10.9.1

Ground speed refers to the speed of a plane relative to the ground. Airspeed refers to the speed a plane can travel relative to its surrounding air mass. These two quantities are not the same because of the effect of wind. In an earlier section, we used triangles to solve a similar problem involving the movement of boats. Later in this section, we will find the airplane’s ground speed and bearing, while investigating another approach to problems of this type. First, however, let’s examine the basics of vectors.

A Geometric View of Vectors

A vector is a specific quantity drawn as a line segment with an arrowhead at one end. It has an initial point, where it begins, and a terminal point, where it ends. A vector is defined by its magnitude, or the length of the line, and its direction, indicated by an arrowhead at the terminal point. Thus, a vector is a directed line segment. There are various symbols that distinguish vectors from other quantities:

  • Lower case, boldfaced type, with or without an arrow on top such as \(u\), \(w\), \(\overrightarrow{v}\), \(\overrightarrow{u}\), \(\overrightarrow{w}\).
  • Given initial point \(P\) and terminal point \(Q\), a vector can be represented as \(\overrightarrow{PQ}\) .  The arrowhead on top is what indicates that it is not just a line, but a directed line segment.
  • Given an initial point of \((0,0)\) and terminal point \((a,b)\), a vector may be represented as \(⟨a,b⟩\).

This last symbol \(⟨a,b⟩\) has special significance. It is called the standard position. The position vector has an initial point \((0,0)\) and a terminal point \(⟨a,b⟩\). To change any vector into the position vector, we think about the change in the x-coordinates and the change in the y-coordinates. Thus, if the initial point of a vector \(\overrightarrow{CD}\) is \(C(x_1,y_1)\) and the terminal point is \(D(x_2,y_2)\), then the position vector is found by calculating

\(\overrightarrow{AB}=⟨x_2−x_1,y_2−y_1⟩\)

\(=⟨a,b⟩\)

 

In Figure 10.9.2, we see the original vector \(\overrightarrow{CD}\) and the position vector \(\overrightarrow{AB}\).

Plot of the original vector CD in blue and the position vector AB in orange extending from the origin.

Figure 10.9.2

A General Note: PROPERTIES OF VECTORS

A vector is a directed line segment with an initial point and a terminal point. Vectors are identified by magnitude, or the length of the line, and direction, represented by the arrowhead pointing toward the terminal point. The position vector has an initial point at \((0,0)\) and is identified by its terminal point \(⟨a,b⟩\).

Example

Find the Position Vector

Consider the vector whose initial point is \(P(2,3)\) and terminal point is \(Q(6,4)\). Find the position vector.

Solution:

The position vector is found by subtracting one x-coordinate from the other x-coordinate, and one y-coordinate from the other y-coordinate. Thus

\(v=⟨6−2,4−3⟩\)

\(=⟨4,1⟩\)

The position vector begins at \((0,0)\) and terminates at \((4,1)\). The graphs of both vectors are shown in Figure 10.9.3.

Plot of the original vector in blue and the position vector in orange extending from the origin.

Figure 10.9.3

We see that the position vector is \(⟨4,1⟩\).

Example

Drawing a Vector with the Given Criteria and Its Equivalent Position Vector

Find the position vector given that vector \(v\) has an initial point at \((−3,2)\) and a terminal point at \((4,5)\), then graph both vectors in the same plane.

Solution:

The position vector is found using the following calculation:

\(v=⟨4−(−3),5−2⟩\)  

\(=⟨7,3⟩\)

Thus, the position vector begins at \((0,0)\) and terminates at \((7,3)\). See Figure 10.9.4.

Plot of the two given vectors their same position vector.

Figure 10.9.4

Exercise 

Draw a vector \(v\) that connects from the origin to the point \((3,5)\).

Solution:

A vector from the origin to (3,5) - a line with an arrow at the (3,5) endpoint.

Finding Magnitude and Direction

To work with a vector, we need to be able to find its magnitude and its direction. We find its magnitude using the Pythagorean Theorem or the distance formula, and we find its direction using the inverse tangent function.

A General Note: MAGNITUDE AND DIRECTION OF A VECTOR

Given a position vector \(v=⟨a,b⟩\),the magnitude is found by \(| v |=\sqrt{a^2+b^2}\).The direction is equal to the angle formed with the x-axis, or with the y-axis, depending on the application. For a position vector, the direction is found by \(\tan \theta=(\dfrac{b}{a})⇒\theta={\tan}^{−1}(\dfrac{b}{a})\), as illustrated in Figure 10.9.5.

Standard plot of a position vector (a,b) with magnitude |v| extending into Q1 at theta degrees.

Figure 10.9.5

Two vectors v and u are considered equal if they have the same magnitude and the same direction. Additionally, if both vectors have the same position vector, they are equal.

Example

Finding the Magnitude and Direction of a Vector

Find the magnitude and direction of the vector with initial point \(P(−8,1)\) and terminal point \(Q(−2,−5)\).Draw the vector.

Solution:

First, find the position vector.

\(u=⟨−2,−(−8),−5−1⟩\)  

\(=⟨6,−6⟩\)

We use the Pythagorean Theorem to find the magnitude.

\(|u|=\sqrt{{(6)}^2+{(−6)}^2}\)      

\(=\sqrt{72}\)      

\(=\sqrt{62}\)

The direction is given as

\(\tan \theta=\dfrac{−6}{6}=−1\rightarrow \theta={\tan}^{−1}(−1)\)            

\(=−45°\)

However, the angle terminates in the fourth quadrant, so we add 360° to obtain a positive angle. Thus, \(−45°+360°=315°\). See Figure 10.9.6.

Plot of the position vector extending into Q4 from the origin with the magnitude 6rad2.

Figure 10.9.6

Example

Showing That Two Vectors Are Equal

Show that vector v with initial point at \((5,−3)\) and terminal point at \((−1,2)\) is equal to vector u with initial point at \((−1,−3)\) and terminal point at \((−7,2)\). Draw the position vector on the same grid as v and u. Next, find the magnitude and direction of each vector.

Solution:

As shown in Figure 10.9.7, draw the vector \(v\) starting at initial \((5,−3)\) and terminal point \((−1,2)\). Draw the vector \(u\) with initial point \((−1,−3)\) and terminal point \((−7,2)\). Find the standard position for each.

Next, find and sketch the position vector for v and u. We have

\(v=⟨−1−5,2−(−3)⟩\)  

\(=⟨−6,5⟩u\)

\(=⟨−7−(−1),2−(−3)⟩\) 

\(=⟨−6,5⟩\)

Since the position vectors are the same, v and u are the same.

An alternative way to check for vector equality is to show that the magnitude and direction are the same for both vectors. To show that the magnitudes are equal, use the Pythagorean Theorem.

\(|v|=\sqrt{{(−1−5)}^2+{(2−(−3))}^2}\)      

\(=\sqrt{{(−6)}^2+{(5)}^2}\)     

 \(=\sqrt{36+25}\)      

\(=\sqrt{61}\)

\(|u|=\sqrt{{(−7−(−1))}^2+{(2−(−3))}^2}\)      

\(=\sqrt{{(−6)}^2+{(5)}^2}\)      

\(=\sqrt{36+25}\)     

 \(=\sqrt{61}\)

As the magnitudes are equal, we now need to verify the direction. Using the tangent function with the position vector gives

\(\tan \theta=−\dfrac{5}{6}⇒\theta={\tan}^{−1}(−\dfrac{5}{6})\)           

\( =−39.8°\)

However, we can see that the position vector terminates in the second quadrant, so we add \(180°\). Thus, the direction is \(−39.8°+180°=140.2°\).

Plot of the two given vectors their same position vector.

Figure 10.9.7

 

Performing Vector Addition and Scalar Multiplication

Now that we understand the properties of vectors, we can perform operations involving them. While it is convenient to think of the vector \(u=⟨x,y⟩\) as an arrow or directed line segment from the origin to the point \((x,y)\), vectors can be situated anywhere in the plane. The sum of two vectors u and v, or vector addition, produces a third vector uv, the resultant vector.

To find u + v, we first draw the vector u, and from the terminal end of u, we drawn the vector v. In other words, we have the initial point of v meet the terminal end of u. This position corresponds to the notion that we move along the first vector and then, from its terminal point, we move along the second vector. The sum u + v is the resultant vector because it results from addition or subtraction of two vectors. The resultant vector travels directly from the beginning of u to the end of v in a straight path, as shown in Figure 10.9.8.

Diagrams of vector addition and subtraction.

Figure 10.9.8

Vector subtraction is similar to vector addition. To find u − v, view it as u + (−v). Adding −v is reversing direction of v and adding it to the end of u. The new vector begins at the start of u and stops at the end point of −v. See Figure 10.9.9 for a visual that compares vector addition and vector subtraction using parallelograms.

Showing vector addition and subtraction with parallelograms. For addition, the base is u, the side is v, the diagonal connecting the start of the base to the end of the side is u+v. For subtraction, thetop is u, the side is -v, and the diagonal connecting the start of the top to the end of the side is u-v.

Figure 10.9.9

Example

Adding and Subtracting Vectors

Given \(u=⟨3,−2⟩\) and \(v=⟨−1,4⟩\), find two new vectors u + v, and u − v.

Solution:

To find the sum of two vectors, we add the components. Thus,

\(u+v=⟨3,−2⟩+⟨−1,4⟩\)

\(=⟨3+(−1),−2+4⟩\)

\(=⟨2,2⟩\)

See Figure 10.9.10 (a).

To find the difference of two vectors, add the negative components of v to u. Thus,

\(u+(−v)=⟨3,−2⟩+⟨1,−4⟩\)

\(=⟨3+1,−2+(−4)⟩\)

\(=⟨4,−6⟩\)

See Figure 10.9.10 (b).

Further diagrams of vector addition and subtraction.

Figure 10.9.10 (a) Sum of two vectors (b) Difference of two vectors

Multiplying By a Scalar

While adding and subtracting vectors gives us a new vector with a different magnitude and direction, the process of multiplying a vector by a scalar, a constant, changes only the magnitude of the vector or the length of the line. Scalar multiplication has no effect on the direction unless the scalar is negative, in which case the direction of the resulting vector is opposite the direction of the original vector.

A General Note: SCALAR MULTIPLICATION

Scalar multiplication involves the product of a vector and a scalar. Each component of the vector is multiplied by the scalar. Thus, to multiply \(v=⟨a,b⟩\) by \(k\), we have

\(kv=⟨ka,kb⟩\)

Only the magnitude changes, unless \(k\) is negative, and then the vector reverses direction.

Example

Performing Scalar Multiplication

Given vector \(v=⟨3,1⟩\), find 3v, \(\dfrac{1}{2}\), and −v.

Solution:

See Figure 10.9.11 for a geometric interpretation. If  \(v=⟨3,1⟩\), then

\(3v=⟨3⋅3,3⋅1⟩\)

\(=⟨9,3⟩\)

\(\dfrac{1}{2}v=⟨\dfrac{1}{2}⋅3,\dfrac{1}{2}⋅1⟩\)

\(=⟨\dfrac{3}{2},\dfrac{1}{2}⟩\)

\(−v=⟨−3,−1⟩\)  

Showing the effect of scaling a vector: 3x, 1x, .5x, and -1x. The 3x is three times as long, the 1x stays the same, the .5x halves the length, and the -1x reverses the direction of the vector but keeps the length the same. The rest keep the same direction; only the magnitude changes.

Figure 10.9.11

Analysis

Notice that the vector 3v is three times the length of v, \(\dfrac{1}{2}v\) is half the length of v, and –v is the same length of v, but in the opposite direction.

Exercise 

Find the scalar multiple \(3u\) given \(u=⟨5,4⟩\).

Solution:

\(3u=⟨15,12⟩\)

Example

Find a linear equation to solve for the following unknown quantities: One number exceeds another number by \( 17\)  and their sum is \( 31\). Find the two numbers.

Solution:

First, we must multiply each vector by the scalar.

\(3u=3⟨3,−2⟩\)

\(=⟨9,−6⟩\)

\(2v=2⟨−1,4⟩\)

\(=⟨−2,8⟩\)

Then, add the two together.

\(w=3u+2v\)

\(=⟨9,−6⟩+⟨−2,8⟩\)

\(=⟨9−2,−6+8⟩\)

\(=⟨7,2⟩\)

So, \(w=⟨7,2⟩\).

Finding Component Form

In some applications involving vectors, it is helpful for us to be able to break a vector down into its components. Vectors are comprised of two components: the horizontal component is the \(x\) direction, and the vertical component is the \(y\) direction. For example, we can see in the graph in Figure 10.9.12 that the position vector \(⟨2,3⟩\) comes from adding the vectors \(v_1\) and \(v_2\). We have \(v_2\) with initial point \((0,0)\) and terminal point \((2,0)\). 

\(v_1=⟨2−0,0−0⟩\)

\(=⟨2,0⟩\)

We also have \(v_2\) with initial point \((0,0)\) and terminal point \((0, 3)\). 

\(v_2=⟨0−0,3−0⟩\)

\(=⟨0,3⟩\)

Therefore, the position vector is

\(v=⟨2+0,3+0⟩\)

\(=⟨2,3⟩\)

Using the Pythagorean Theorem, the magnitude of \(v_1\) is \(2\), and the magnitude of \(v_2\) is \(3\). To find the magnitude of \(v\), use the formula with the position vector.

\(|v|=\sqrt{{|v_1|}^2+{|v_2|}^2}\)     

\(=\sqrt{2^2+3^2}\)     

\(=\sqrt{13}\)

The magnitude of \(v\) is \(\sqrt{13}\). To find the direction, we use the tangent function \(\tan \theta=\dfrac{y}{x}\).

\(\tan \theta=\dfrac{v_2}{v_1}\)

\(\tan \theta=\dfrac{3}{2}\)         

\(\theta={\tan}^{−1}(\dfrac{3}{2})=56.3°\)

Diagram of a vector in root position with its horizontal and vertical components.

Figure 10.9.12

Thus, the magnitude of \(v\) is \(\sqrt{13}\) and the direction is \(56.3∘\) off the horizontal.

Example

Finding the Components of the Vector

Find the components of the vector \(v\) with initial point \((3,2)\) and terminal point \((7,4)\).

Solution:

First find the standard position.

\(v=⟨7−3,4−2⟩\)

\(=⟨4,2⟩\)

See the illustration in Figure 10.9.13.

Diagram of a vector in root position with its horizontal (4,0) and vertical (0,2) components.

Figure 10.9.13

The horizontal component is \(v_1=⟨4,0⟩\) and the vertical component is \(v_2=⟨0,2⟩\).

Finding the Unit Vector in the Direction of v

In addition to finding a vector’s components, it is also useful in solving problems to find a vector in the same direction as the given vector, but of magnitude 1. We call a vector with a magnitude of 1 a unit vector. We can then preserve the direction of the original vector while simplifying calculations.

Unit vectors are defined in terms of components. The horizontal unit vector is written as \(i=⟨1,0⟩\) and is directed along the positive horizontal axis. The vertical unit vector is written as \(j=⟨0,1⟩\) and is directed along the positive vertical axis. See Figure 10.9.14.

Plot showing the unit vectors i=91,0) and j=(0,1)

Figure 10.9.14

A General Note: THE UNIT VECTORS

If \(v\) is a nonzero vector, then \(\dfrac{v}{| v |}\) is a unit vector in the direction of \(v\). Any vector divided by its magnitude is a unit vector. Notice that magnitude is always a scalar, and dividing by a scalar is the same as multiplying by the reciprocal of the scalar.

Example

Finding the Unit Vector in the Direction of v

Find a unit vector in the same direction as \(v=⟨−5,12⟩\).

Solution:

First, we will find the magnitude.

\(|v|=\sqrt{{(−5)}^2+{(12)}^2}\)      

\(=\sqrt{25+144}\)      

\(=\sqrt{169}\)      

\(=13\)

Then we divide each component by \(| v |\), which gives a unit vector in the same direction as v:

\(\dfrac{v}{| v |}=−\dfrac{5}{13}i+\dfrac{12}{13}j\)

or, in component form

\(\dfrac{v}{| v |}=⟨ −\dfrac{5}{13},\dfrac{12}{13} ⟩\)

See Figure 10.9.15.

Plot showing the unit vector (-5/13, 12/13) in the direction of (-5, 12)

Figure 10.9.15

Verify that the magnitude of the unit vector equals 1. The magnitude of \(−\dfrac{5}{13}i+\dfrac{12}{13}j\) is given as

\(\sqrt{ {(−\dfrac{5}{13})}^2+{ (\dfrac{12}{13}) }^2 }=\sqrt{\dfrac{25}{169}+\dfrac{144}{169}}\)                            

\(=\sqrt{\dfrac{169}{169}}=1\)

The vector \(u=\dfrac{5}{13}i+\dfrac{12}{13}j\) is the unit vector in the same direction as \(v=⟨−5,12⟩\).

Performing Operations with Vectors in Terms of i and j

So far, we have investigated the basics of vectors: magnitude and direction, vector addition and subtraction, scalar multiplication, the components of vectors, and the representation of vectors geometrically. Now that we are familiar with the general strategies used in working with vectors, we will represent vectors in rectangular coordinates in terms of i and j.

A General Note: VECTORS IN THE RECTANGULAR PLANE

Given a vector\(v\)with initial point \(P=(x_1,y_1)\) and terminal point \(Q=(x_2,y_2)\), v is written as

\(v=(x_2−x_1)i+(y_1−y_2)j\)

The position vector from \((0,0)\) to \((a,b)\), where \((x_2−x_1)=a\) and \((y_2−y_1)=b\), is written as v = aibj. This vector sum is called a linear combination of the vectors i and j.

The magnitude of v = ai + bj is given as \(| v |=\sqrt{a^2+b^2}\). See Figure 10.9.16.

Plot showing vectors in rectangular coordinates in terms of i and j. The position vector v (in orange) extends from the origin to some point (a,b) in Q1. The horizontal (ai) and vertical (bj) components are shown.

Figure 10.9.16

Example

Writing a Vector in Terms of i and j

Given a vector v with initial point \(P=(2,−6)\) and terminal point \(Q=(−6,6)\), write the vector in terms of i and j.

Solution:

Begin by writing the general form of the vector. Then replace the coordinates with the given values.

\(v=(x_2−x_1)i+(y_2−y_1)j\)   

\(=(−6−2)i+(6−(−6))j\)   

\(=−8i+12j\)

Example

Writing a Vector in Terms of i and j Using Initial and Terminal Points

Given initial point \(P_1=(−1,3)\) and terminal point \(P_2=(2,7)\), write the vector v in terms of i and j. 

Solution:

Begin by writing the general form of the vector. Then replace the coordinates with the given values.

\(v=(x_2−x_1)i+(y_2−y_1)j\)

\(v=(2−(−1))i+(7−3)j\)  

\(=3i+4j\)

Exercise 

Write the vector u with initial point \(P=(−1,6)\) and terminal point \(Q=(7,−5)\) in terms of i and j.

Solution:

\(u=8i−11j\)

Performing Operations on Vectors in Terms of i and j

When vectors are written in terms of \(i\) and \(j\), we can carry out addition, subtraction, and scalar multiplication by performing operations on corresponding components.

A General Note: ADDING AND SUBTRACTING VECTORS IN RECTANGULAR COORDINATES

Given \(v = ai + bj\) and \(u = ci + dj\), then

\(v+u=(a+c)i+(b+d)j\)

\(v−u=(a−c)i+(b−d)j\)

Example

Finding the Sum of the Vectors

Find the sum of \(v_1=2i−3j\) and \(v_2=4i+5j\).

Solution:

\(v_1+v_2=(2+4)i+(−3+5)j\)                 

\(=6i+2j\)

Calculating the Component Form of a Vector: Direction

We have seen how to draw vectors according to their initial and terminal points and how to find the position vector. We have also examined notation for vectors drawn specifically in the Cartesian coordinate plane using \(i\) and \(j\). For any of these vectors, we can calculate the magnitude. Now, we want to combine the key points, and look further at the ideas of magnitude and direction.

Calculating direction follows the same straightforward process we used for polar coordinates. We find the direction of the vector by finding the angle to the horizontal. We do this by using the basic trigonometric identities, but with \(| v |\) replacing \(r\).

A General Note: VECTOR COMPONENTS IN TERMS OF MAGNITUDE AND DIRECTION

Given a position vector \(v=⟨x,y⟩\)  and a direction angle \(\theta\),

\(\cos \theta=\dfrac{x}{|v|}\) and \(\sin \theta=y|v|        

\(x=|v| \cos \theta\)      \(y=|v| \sin \theta\)

Thus, \(v=xi+yj=| v | \cos \theta i+| v | \sin \theta j\), and magnitude is expressed as \(| v |=\sqrt{x^2+y^2}\).

Example

 

Writing a Vector in Terms of Magnitude and Direction

Write a vector with length 7 at an angle of 135° to the positive x-axis in terms of magnitude and direction.

Solution:

Using the conversion formulas \(x=| v | \cos \theta i\) and \(y=| v | \sin \theta j\), we find that

\(x=7\cos(135°)i\)   

\(=−\dfrac{7\sqrt{2}}{2}\)

\(y=7 \sin(135°)j\)   

\(=\dfrac{7\sqrt{2}}{2}\)

This vector can be written as \(v=7\cos(135°)i+7\sin(135°)j\) or simplified as

\(v=−\dfrac{7\sqrt{2}}{2}i+\dfrac{7\sqrt{2}}{2}j\)

Exercise 

A vector travels from the origin to the point \((3,5)\). Write the vector in terms of magnitude and direction.

Solution:

\(v=\sqrt{34}\cos(59°)i+\sqrt{34}\sin(59°)j\)

Magnitude = 34

\(\theta={\tan}^{−1}(\dfrac{5}{3})=59.04°\)

Finding the Dot Product of Two Vectors

As we discussed earlier in the section, scalar multiplication involves multiplying a vector by a scalar, and the result is a vector. As we have seen, multiplying a vector by a number is called scalar multiplication. If we multiply a vector by a vector, there are two possibilities: the dot product and the cross product. We will only examine the dot product here; you may encounter the cross product in more advanced mathematics courses.

The dot product of two vectors involves multiplying two vectors together, and the result is a scalar.

A General Note: DOT PRODUCT

The dot product of two vectors \(v=⟨a,b⟩\) and \(u=⟨c,d⟩\) is the sum of the product of the horizontal components and the product of the vertical components.

\(v⋅u=ac+bd\)

To find the angle between the two vectors, use the formula below.

\(\cos \theta=\dfrac{v}{| v |}⋅\dfrac{u}{| u |}\)

Example

Finding the Dot Product of Two Vectors

Find the dot product of \(v=⟨5,12⟩\) and \(u=⟨−3,4⟩\). 

Solution:

Using the formula, we have

\(v⋅u=⟨5,12⟩⋅⟨−3,4⟩\)

\(=5⋅(−3)+12⋅4\)

\(=−15+48\)

\(=33\)

Example

Finding the Dot Product of Two Vectors and the Angle between Them

Find the dot product of  \(v_1 = 5i + 2j\) and \(v_2 = 3i + 7j\). Then, find the angle between the two vectors.

Solution:

Finding the dot product, we multiply corresponding components.

\(v_1⋅v_2=⟨5,2⟩⋅⟨3,7⟩\)

\(=5⋅3+2⋅7\)

\(=15+14\)

\(=29\)

 

To find the angle between them, we use the formula \(\cos \theta=\dfrac{v}{|v|}⋅\dfrac{u}{|u|}\).          

\(\dfrac{v}{|v|}⋅\dfrac{u}{|u|}=⟨ \dfrac{5}{\sqrt{29}}+\dfrac{2}{\sqrt{29}} ⟩⋅⟨ \dfrac{3}{\sqrt{58}}+\dfrac{7}{\sqrt{58}} ⟩\)                                         

\(=\dfrac{5}{\sqrt{29}}⋅\dfrac{3}{\sqrt{58}}+\dfrac{2}{\sqrt{29}}⋅\dfrac{7}{\sqrt{58}}\)                                         

\(=\dfrac{15}{\sqrt{1682}}+\dfrac{14}{\sqrt{1682}}=\dfrac{29}{\sqrt{1682}}\)                                         

\(=0.707107\)

\({\cos}^{−1}(0.707107)=45°\)

See Figure 10.9.17.

Plot showing the two position vectors (3,7) and (5,2) and the 45 degree angle between them.

Figure 10.9.17

Example

Finding the Angle between Two Vectors

Find the angle between \(u=⟨−3,4⟩\) and \(v=⟨5,12⟩\).

Solution:

Using the formula, we have

\(\theta={\cos}^{−1}(\dfrac{u}{|u|}⋅\dfrac{v}{|v|})\)

\((\dfrac{u}{|u|}⋅\dfrac{v}{|v|})=\dfrac{−3i+4j}{5}⋅\dfrac{5i+12j}{13}\)                      

\(=(− \dfrac{3}{5}⋅ \dfrac{5}{13})+(\dfrac{4}{5}⋅ \dfrac{12}{13})\)                      

\(=−\dfrac{15}{65}+\dfrac{48}{65}\)                     

\(=\dfrac{33}{65}\)                   

\(\theta={\cos}^{−1}(\dfrac{33}{65})\)                      

\(=59.5∘\)

See Figure 10.9.18.

Plot showing the two position vectors (-3,4) and (5,12) and the 59.5 degree angle between them.

Figure 10.9.18

Example

Finding Ground Speed and Bearing Using Vectors

We now have the tools to solve the problem we introduced in the opening of the section.

An airplane is flying at an airspeed of 200 miles per hour headed on a SE bearing of 140°. A north wind (from north to south) is blowing at 16.2 miles per hour. What are the ground speed and actual bearing of the plane? See Figure 10.9.19.

Image of a plan flying SE at 140 degrees and the north wind blowing.

Figure 10.9.19

Solution:

The ground speed is represented by \(x\) in the diagram, and we need to find the angle \(\alpha\) in order to calculate the adjusted bearing, which will be \(140°+\alpha\) .

Notice in Figure 10.9.19, that angle \(BCO\) must be equal to angle \(AOC\) by the rule of alternating interior angles, so angle \(BCO\) is 140°. We can find \(x\) by the Law of Cosines:

\(x^2={(16.2)}^2+{(200)}^2−2(16.2)(200) \cos(140°)\)

\(x^2=45,226.41\)   

\(x=\sqrt{45,226.41}\)   

\(x=212.7\)

The ground speed is approximately 213 miles per hour. Now we can calculate the bearing using the Law of Sines.                  

\(\dfrac{\sin \alpha}{16.2}=\dfrac{\sin(140°)}{212.7}\)                          

\(\sin \alpha=\dfrac{16.2 \sin(140°)}{212.7}\)                                     

\(=0.04896\)

\({\sin}^{−1}(0.04896)=2.8°\)

Therefore, the plane has a SE bearing of 140°+2.8°=142.8°. The ground speed is 212.7 miles per hour.

Media: Access these online resources for additional instruction and practice with vectors.

Key Concepts

  • The position vector has its initial point at the origin. See Example.
  • If the position vector is the same for two vectors, they are equal. See Example.
  • Vectors are defined by their magnitude and direction. See Example.
  • If two vectors have the same magnitude and direction, they are equal. See Example.
  • Vector addition and subtraction result in a new vector found by adding or subtracting corresponding elements. See Example.
  • Scalar multiplication is multiplying a vector by a constant. Only the magnitude changes; the direction stays the same. SeeExample and Example.
  • Vectors are comprised of two components: the horizontal component along the positive x-axis, and the vertical component along the positive y-axis. See Example.
  • The unit vector in the same direction of any nonzero vector is found by dividing the vector by its magnitude.
  • The magnitude of a vector in the rectangular coordinate system is \(| v |=\sqrt{a^2+b^2}\). See Example.
  • In the rectangular coordinate system, unit vectors may be represented in terms of ii and jj wherei i represents the horizontal component andj j represents the vertical component. Then, v = ai + bj  is a scalar multiple of v by real numbers a and b. SeeExample and Example.
  • Adding and subtracting vectors in terms of i and j consists of adding or subtracting corresponding coefficients of i and corresponding coefficients of j. See Example.
  • A vector v = ai + bj is written in terms of magnitude and direction as \(v=| v |\cos \theta i+| v |\sin \theta j\). See Example.
  • The dot product of two vectors is the product of thei i terms plus the product of the j terms. See Example.
  • We can use the dot product to find the angle between two vectors. Example and Example.
  • Dot products are useful for many types of physics applications. See Example.