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Polar Coordinates

(r, theta) is a point based on distance from the origin and angle on the unit circle.

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Polar Coordinates

Everyone has dreamed of flying at one time or another. Not only would there be much less traffic to worry about, but directions would be so much simpler!

Walking or driving: "Go East 2 blocks, turn left, then North 6 blocks. Wait for the train. Turn right, East 3 more blocks, careful of the cow! Turn left, go North 4 more blocks and park."

Flying: "Fly 30 deg East of North for a little less than 11 and 1/4 blocks. Land."

Nice daydream, but what does it have to do with polar coordinates?

Polar Coordinates

The polar coordinate system is an alternative to the Cartesian coordinate system you have used in the past to graph functions. The polar coordinate system is specialized for visualizing and manipulating angles.

Angles are identified by travelling counter-clockwise around the circular graph from the 0 deg line, or r-axis (where the + x axis would be) to a specified angle.

To plot a specific point, first go along the r-axis by r units. Then, rotate counterclockwise by the given angle, commonly represented "θ". Be careful to use the correct units for the angle measure (either radians or degrees).

Radians

Usually polar plots are done with radians (especially if they include trigonometric functions), but sometimes degrees are used.

A radian is the angle formed between the r axis and a polar axis drawn to meet a section of the circumference that is the same length as the radius of a circle.

Given that the circumference of a circle is 2πr, and since r is the radius, that means there are 2π radians in a complete circle, and 1π radians in 1/2 of a circle.

If 1/2 of a circle is π radians, and is 180 deg, that means that there are 180π degrees in each radian.

That translates to approximately 57.3 degrees = 1 radian.

Graphing Using Technology

Polar equations can be graphed using a graphing calculator: With the graphing calculator- go to MODE. There select RADIAN for the angle measure and POL (for Polar) on the FUNC (function)line. When Y = is pressed, note that the equation has changed from y = to r = . There input the polar equation. After pressing graph, if you can’t see the full graph, adjust x- and y- max/min, etc in WINDOW.

  

Examples

Example 1

Plot the points on a polar coordinate graph: Point A (2,π3), Point B (4,135o), and Point C (2,π6).

Below is the pole, polar axis and the points A, B and C.

Example 2

Plot the following points.

  1. (4,30o)
  2. (2.5,π)
  3. (1,π3)
  4. (3,5π6)
  5. (2,300o)

Example 3

Use a graphing calculator or plotting program to plot the following equations.

  1. r=1+3sinθ

  1. r=1+2cosθ

Review the steps above under graphing using technology if you are having trouble.

Example 4

Convert from radians to degrees.

Recall that πrad=180o and 1rad=180π57.3o.

  1. π2

If πrad=180o then π2rad=90o

  1. 5.17

If 1rad57.3o then 5.17rad296o

  1. 3π2

If πrad=180o then 3π2rad=270o

Example 5

Convert from degrees to radians.

Recall that 180oπ=57.3o1rad.

  1. 251o

If 57.3o1rad then 251o4.38rad1.4πrad

  1. 360o

If 57.3o1rad then 360o6.28rad

  1. 327o

If 57.3o1rad then 327o57.3o5.71rad

Example 6

Convert from degrees to radians, answer in terms of π.

Recall that 2πrad=360o and therefore πrad=180o.

  1. 90o

If πrad=180o then π2rad=90o

  1. 270o

If πrad=180o and π2rad=90o then 112πrad32π3π2rad=270o

  1. 45o

If π2rad=90o then π4rad=45o

Review

  1. Why can a point on the plane not be labeled using a unique ordered pair (r,θ).
  2. Explain how to graph \begin{align*}(r, \theta)\end{align*} if \begin{align*}r < 0\end{align*} and/or \begin{align*}\theta > 360\end{align*}.

Graph each point on the polar plane.

  1. A \begin{align*}(6, 145^o)\end{align*}
  2. B \begin{align*}\left(-2, \frac{13\pi}{6} \right)\end{align*}
  3. C \begin{align*}\left(\frac{7}{4}, -210^o\right)\end{align*}
  4. D \begin{align*}\left(5, \frac{\pi}{2}\right)\end{align*}
  5. E \begin{align*}\left(3.5, \frac{-\pi}{8}\right)\end{align*}

Name two other pairs of polar coordinates for each point.

  1. \begin{align*}(1.5, 170^o)\end{align*}
  2. \begin{align*}\left(-5, \frac{\pi}{-3}\right)\end{align*}
  3. \begin{align*}(3, 305^o)\end{align*}

Graph each polar equation.

  1. \begin{align*}r = 3\end{align*}
  2. \begin{align*}\theta = \frac{\pi}{5}\end{align*}
  3. \begin{align*}r = 15.5\end{align*}
  4. \begin{align*}r = 1.5\end{align*}
  5. \begin{align*}\theta = -175^o\end{align*}

Find the distance between the given points.

  1. \begin{align*}P_1 \left(5, \frac{\pi}{2}\right)\end{align*} and \begin{align*}P_2 \left(7, \frac{3\pi}{9}\right)\end{align*}
  2. \begin{align*}P_1 (1.3, -52^o) \end{align*} and \begin{align*}P_2 (-13.6, -162^o)\end{align*}
  3. \begin{align*}P_1 (3, 250^o)\end{align*} and \begin{align*} P_2 (7, 90^o)\end{align*}

Review (Answers)

To see the Review answers, open this PDF file and look for section 4.1. 

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Vocabulary

\pi

\pi (Pi) is the ratio of the circumference of a circle to its diameter. It is an irrational number that is approximately equal to 3.14.

Cartesian coordinate system

The Cartesian plane is a grid formed by a horizontal number line and a vertical number line that cross at the (0, 0) point, called the origin.

polar axis

The polar axis is a ray drawn from the pole at the 0^\circ angle on a polar graph.

polar coordinate system

The polar coordinate system is a special coordinate system in which the location of each point is determined by its distance from the pole and its angle with respect to the polar axis.

pole

The pole is the center point on a polar graph.

radian

A radian is a unit of angle that is equal to the angle created at the center of a circle whose arc is equal in length to the radius.

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