10.0: Polar Coordinates

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The Cartesian coordinate system is also called the rectangular coordinate system, because it describes a location in the plane as the vertex of a rectangle. To construct a rectangular coordinate system, we begin with two perpendicular axes that intersect at the origin. The $x-$ and $y$-coordinates of a point indicate the length and width of a rectangle with one vertex at the origin. The point $P(x, y)$ sits at the opposite vertex of the rectangle, as shown in the figure below.

In Chapter 9 we used vectors to specify a location by giving a distance and a direction. For example, we might say that the airport is located 5 miles southwest of the city center. This method of designating locations is so useful that we will construct a new coordinate system using the same tools: distance and direction.

$Screen Shot 2023-01-29 at 12.59.32 AM.png$

In the polar coordinate system we start with the origin or pole, and a single ray from the pole, called the polar axis. We describe the location of a point $P$ in the plane by measuring the distance, $|r|$, from $P$ to the pole, and the angle, $\theta$, that $\overrightarrow{O P}$ makes with the polar axis (measured counterclockwise). The components of the ordered pair $(r, \theta)$ are called the polar coordinates of the point $P$.

Polar Coordinates.

The polar coordinates of a point $P$ in the plane are $(r, \theta)$, where

$|r|$ is the distance from $P$ to the pole,
$\theta$ is the angle measured counterclockwise from the polar axis to the ray through $P$ from the pole.

In our work with polar coordinates, we will always use radians for the angle $\theta$. For example, the point $P\left(2, \frac{\pi}{2}\right)$ is located 2 units from the pole at an angle of $\frac{\pi}{2}$ radians, and $Q(3,4)$ is located 3 units from the pole at an angle of 4 radians. The graphs of $P$ and $Q$ are shown below.

$Screen Shot 2023-01-29 at 1.07.05 AM.png$

A piece of graph paper in rectangular coordinates consists of a grid of horizontal and vertical lines. These are the lines $x=k$ and $y=k$, for evenly spaced values of $k$. Each vertical grid line consists of points that have the same $x$-coordinate, and each horizontal grid line is made up of points that have the same $y$-coordinate. The grid lines are bench marks to help us locate points with specific rectangular coordinates.

A piece of graph paper in polar coordinates consists of a grid of concentric circles and radial lines, as shown below. Each circle consists of points with the same $r$-coordinate; they are all the same distance from the pole. All points with the same $\theta$-coordinate lie on one of the radial lines. This grid helps us locate points with specific polar coordinates.

$Screen Shot 2023-01-29 at 1.09.02 AM.png$

Plotting Points

You are used to thinking in rectangular coordinates: to locate a point, we move so many units left or right, and so many units up or down. When working in polar coordinates, we want to ”think radially:” how far to move from the pole, and in which direction.

Example 10.1

Plot the points whose polar coordinates are given: $A\left(1, \dfrac{\pi}{2}\right)$ and $B\left(2, \dfrac{7 \pi}{4}\right)$.

Answer

To plot point $A$, we move 1 unit away from the pole in the direction $\frac{\pi}{2}$, as shown below. To plot point $B$, we move 2 units away from the pole in the direction $\frac{7 \pi}{4}$.

$Screen Shot 2023-01-29 at 1.14.58 AM.png$

Caution 10.2

Note that even though the independent or input variable in polar coordinates is nearly always $\theta$, it is customary to list the dependent or output variable first: $(r, \theta)$.

Checkpoint 10.3

Give the polar coordinates of the points $S$ and $T$ shown below, with $0 \leq \theta \leq 2 \pi$.

$Screen Shot 2023-01-29 at 1.19.21 AM.png$

Answer: $\quad S(2, \pi), T\left(3, \dfrac{2 \pi}{3}\right)$

Each point has infinitely many polar coordinates, because we can add multiples of $2 \pi$ to the value of $\theta$. For example, the point with polar coordinates $\left(1, \frac{\pi}{2}\right)$ also has polar coordinates $\left(1, \frac{5 \pi}{2}\right)$ and $\left(1, \frac{-3 \pi}{2}\right)$.

If we allow negative values of $r$, there are even more ways to write the coordinates of a point. Each angle designates a line through the pole, and each line has both a positive and a negative direction.

On the line at $\theta=\frac{\pi}{4}$, for instance, the positive direction lies in the first quadrant, so that the point $\left(2, \frac{\pi}{4}\right)$ is designated by $P$ in the figure below. To plot the point $\left(-2, \frac{\pi}{4}\right)$, we move in the opposite direction from the pole, arriving at point $Q$. Note that the point $Q$ can also be designated by the coordinates $\left(2, \frac{5 \pi}{4}\right)$.

$Screen Shot 2023-01-29 at 1.21.13 AM.png$

Non-Uniqueness of Polar Coordinates.

1 Any point with polar coordinates $(r, \theta)$ also has coordinates $(r, \theta+2 k \pi)$, where $k$ is an integer.

2 The point $(r, \theta)$ can also be designated by $(-r, \theta+\pi)$.

3 The pole has coordinates $(0, \theta)$, for any value of $\theta$.

Example 10.4

Give polar coordinates with negative $r$-values for the points in the previous exercise.

Answer: The point $S(2, \pi)$ is also $(-2, \pi)$ or $(-2,0)$. The point $T\left(3, \frac{2 \pi}{3}\right)$ can also be designated by $\left(-3, \frac{2 \pi}{3}+\pi\right)$, or $\left(-3, \frac{5 \pi}{3}\right)$.

Checkpoint 10.5

Give polar coordinates with positive $r$-values and $0 \leq \theta \leq 2 \pi$ for each point given in polar coordinates.

a $\left(-2.5, \dfrac{\pi}{6}\right)$

b $(-0.7,5)$

Answer

a $\left(2.5, \dfrac{7 \pi}{6}\right)$

b $(0.7,5-\pi)$

Regions in the Plane

In Cartesian coordinates, we use equations and inequalities to describe regions in the plane. For example, the shaded region in figure (a) is the graph of $y \leq x+2$. The pair of inequalities $x \geq -2,1<y<3$ describes the region in figure (b). This region is particularly simple, in that its boundaries are portions of the coordinate grid lines $x=k$ and $y=k$, where $k$ is a constant.

$Screen Shot 2023-01-30 at 11.14.52 AM.png$

In the polar plane, the coordinate grid lines are circles centered at the pole, with equations $r = k$, and lines through the pole, with equations $\theta = k$.

Example 10.6

Sketch the region designated by each set of inequalities.

a $1 \leq r < 3$

b $0 \leq r \leq 2, \dfrac{\pi}{6} < \theta < \dfrac{\pi}{3}$

Answer

a The region consists of all points between 1 (inclusive) and 3 units from the pole. No restriction is given on $\theta$, so the region is the annular ring shown in figure (a).

$Screen Shot 2023-01-30 at 11.18.14 AM.png$

b The region includes all points up to 2 units from the pole, but only between the lines $\theta = \dfrac{\pi}{6}$ and $\theta = \dfrac{\pi}{3}$. These curves bound the pie-shaped wedge (circuilar sector) shown in figure (b).

Checkpoint 10.7

Write inequalities to describe each region shown below.

$Screen Shot 2023-01-30 at 11.21.07 AM.png$

Answer

a $r \geq 2, \dfrac{3 \pi}{4}<\theta \leq \dfrac{5 \pi}{4}$

b $2<r<3,0<\theta<\pi$

Converting Between Polar and Cartesian Coordinates

Using trigonometry, it is easy to convert from polar coordinates to Cartesian, or vice versa. The figure below shows how the two sets of coordinates are related.

If we know the polar coordinates $(r, \theta)$, we can compute the Cartesian coordinates $(x, y)$ as the legs of a right triangle.

\begin{aligned}
& \dfrac{x}{r}=\cos \theta, \quad \text { so } \quad x=r \cos \theta \\
& \dfrac{y}{r}=\sin \theta, \quad \text { so } \quad y=r \sin \theta \\
\end{aligned}

$Screen Shot 2023-01-30 at 11.26.09 AM.png$

Example 10.8

Find the Cartesian coordinates of the point with polar coordinates $\left(\sqrt{2}, \frac{3 \pi}{4}\right)$.

Answer

The polar coordinates tell us that $r=\sqrt{2}$ and $\theta=\frac{3 \pi}{4}$, as shown below.

The $x$-coordinate of the point is

\begin{aligned}
x & =r \cos \theta=\sqrt{2} \cos \left(\dfrac{3 \pi}{4}\right) \\
& =\sqrt{2} \cdot \dfrac{-1}{\sqrt{2}}=-1
\end{aligned}

$Screen Shot 2023-01-30 at 11.28.24 AM.png$

and its $y$-coordinate is

$y=r \sin \theta=\sqrt{2} \sin \left(\dfrac{3 \pi}{4}\right)=\sqrt{2} \cdot \dfrac{1}{\sqrt{2}}=1$

Thus, the Cartesian coordinates are (−1, 1).

Checkpoint 10.9

Find the Cartesian coordinates of the point with polar coordinates $\left(4, \frac{5 \pi}{3}\right)$.

Answer: $(2,-2 \sqrt{3})$

If we know the Cartesian coordinates of a point, we can compute the polar coordinates as follows.

\begin{gathered}
x^2+y^2=r^2, \text { so } r=\sqrt{x^2+y^2} \\
\tan \theta=\dfrac{y}{x}
\end{gathered}

To find the angle $\theta$, it is not enough to compute $\tan ^{-1}\left(\frac{y}{x}\right)$; we must choose the angle in the same quadrant as the given point.

Example 10.10

Find the polar coordinates of the point with Cartesian coordinates $\left(-\dfrac{1}{2},-\dfrac{\sqrt{3}}{2}\right)$.

Answer

We first compute the value of $r$.

$r^2=x^2+y^2=\left(-\dfrac{1}{2}\right)^2+\left(-\dfrac{\sqrt{3}}{2}\right)^2=\dfrac{1}{4}+\dfrac{3}{4}=1$

Thus, $r=\sqrt{1}=1$, as shown at right. To find, we compute

$\tan \theta=\dfrac{y}{x}=\dfrac{-\frac{\sqrt{3}}{2}}{-\frac{1}{2}}=\sqrt{3}$

There are infinitely many values of $\theta$ that satisfy this equation, but our point lies in the third quadrant, so we take $\theta=\frac{4 \pi}{3}$.

$Screen Shot 2023-01-30 at 11.38.19 AM.png$

You can check that the polar coordinates $\left(1, \frac{4 \pi}{3}\right)$ convert back to Cartesian coordinates

$\left(-\dfrac{1}{2},-\dfrac{\sqrt{3}}{2}\right)$

Caution 10.11

In the previous Example, the angle $\theta$ is not given by $\tan ^{-1}(\sqrt{3})=\frac{\pi}{3}$, because the given point lies in the third quadrant. Remember that there are always two angles between 0 and $2 \pi$ with a given tangent.

Checkpoint 10.12

Find polar coordinates for the points with Cartesian coordinates (2, −5). Give decimal approximations rounded to hundredths.

Answer: (5.39, 5.09)

Conversion Equations.

1 To convert from polar coordinates $(r, \theta)$ to Cartesian:

\begin{aligned}
& x=r \cos \theta \\
& y=r \sin \theta
\end{aligned}

2 To convert from Cartesian coordinates $(x, y)$ to polar:

\begin{aligned}
& r=\sqrt{x^2+y^2} \\
& \tan \theta=\dfrac{y}{x}
\end{aligned}

where the choice of $\theta$ depends on the quadrant.

Equations in Polar Coordinates

Polar coordinates are useful for studying objects or phenomena that have radial symmetry, such as circles, spheres, and cylinders, or the central forces (those that act equally in all directions), such as gravity and electric charge. The equations describing these objects are often simpler in polar coordinates than they are in Cartesian coordinates.

It is not hard to convert an equation from Cartesian to polar coordinates: we replace each $x$ with $r \cos \theta$ and each $y$ with $r \sin \theta$.

Example 10.13

Convert the equation $2 x+3 y=6$ to polar coordinates.

Answer

Replacing by $x$ by $r \cos \theta$ and $y$ by $r \sin \theta$, we get

$2 r \cos \theta+3 r \sin \theta=6$

If we solve for $r$ in terms of $\theta$, we find

\begin{aligned}
& r(2 \cos \theta+3 \sin \theta)=6 \\
& r=\dfrac{6}{2 \cos \theta+3 \sin \theta}
\end{aligned}

Neither of these equations is simpler than the Cartesian version, but that should not surprise us, because the original equation describes a line, and Cartesian coordinates are better suited to rectilinear graphs.

Checkpoint 10.14

Convert the equation $x^2+y^2=4$ to polar coordinates.

Answer: $r=2$

Perhaps you recognize the equation in the previous exercise as a circle. Note also that its polar equation has a simpler form than the familiar Cartesian version.

Converting equations from polar to Cartesian form is not as straightforward as the other direction (and is sometimes impossible). Here are some strategies to try:

Expressions of the form $r \cos \theta$ or $r \sin \theta$ can be replaced by $x$ and $y$, respectively.
$r^2$ can be replaced by $x^2+y^2$.
$\tan \theta$ can be replaced by $\frac{y}{x}$.

Example 10.15

Convert the equation $r=3 \cos \theta$ to Cartesian coordinates.

Answer

We multiply both sides of the equation by $r$ to obtain

$r^2=3 r \cos \theta$

Now we can replace the left side by $x^2+y^2$ and the right side by $3 x$ to obtain

$x^2+y^2=3 x$

This is the equation of a circle. By completing the square in each variable, we can write the equation in standard form,

$\left(x-\dfrac{3}{2}\right)^2+y^2=\dfrac{9}{4}$

The center of the circle is $\left(\dfrac{3}{2}, 0\right)$, and its radius is $\dfrac{3}{2}$.

Checkpoint 10.16

Convert the equation $r=\tan \theta$ to Cartesian coordinates.

Answer: $y^2=\dfrac{x^4}{1-x^2}$

Review the following skills you will need for this section.

Algebra Refreshers 10.1

Recall the standard form for the equation of a circle of radius $r$ centered at $(h, k)$:

$(x-h)^2+(y-k)^2=r^2$

Use completing the square to write each equation in standard form.

1 $x^2+y^2-4 x+2 y-4=0$

2 $x^2+y^2=88+18 y$

3 $x^2+y^2+25=9-10 x-9 y$

4 $5 y^2=85-5 x^2-10 x$

Algebra Refresher Answers

1 $(x-2)^2+(y+1)^2=3^2$

2 $x^2+(y-9)^2=13^2$

3 $(x-5)^2+(y-3)^2=(3 \sqrt{2})^2$

4 $(x-1)^2+y^2=4^2$

Section 10.1 Summary

Vocabulary

• Pole

• Polar axis

• Polar coordinates