< Previous Section | Home | Next Section>> |

This section assumes that you have learned about radian angle measure. Review Chapter 2 if you are unfamiliar with radian angle measure.

A two-dimensional coordinate system is a method for locating points in the plane. It is also a method for representing the values of two quantities simultaneously. It was by Descartes’ ingenious method of representing the location of points in a plane or in space that we can visually represent relationships between quantities’ values.

A rectangular coordinate systems is like a street map of Phoenix (an exceptionally flat city). Locations are determined by intersections of lines that are perpendicular to axes (see Section 3.9).

A polar system works like radar. A
point’s location is recorded according to its distance from a * reference
point* and its angle from a

Figure 3.9.3 illustrates the polar coordinate system. Angles are measured
in radians (see the Sections 2.2,
2.3, and 2.4).
Distances from the pole are measured in units of the radius of the unit
circle. Coordinates in the polar plane are customarily in the form (*r*,
θ), where *r* is the point’s distance from the pole and θ is the
measure in radians of the angle formed by a ray from the pole to the point
and the reference direction. A point with coordinates (3, 1.2) in the
polar system is 3 unit radius lengths from the pole in a direction of 1.2
radians from the polar axis.

*Figure 3.9.3 The polar coordinate system. Rays point in directions,
measured in radians from the polar axis. Circles designate distances
from the pole, measured in units of the radius of the unit circle.
Coordinates of a point are given as (distance from pole, direction from
polar axis).*

The animation in Figure 3.9.4 shows a way to think about determining a point’s location in the polar plane. First determine its distance from the pole. Once the distance is settled, determine the direction from the polar axis.

*Figure 3.9.4. Locate a point in the polar
coordinate system by first determining its distance from the pole and
then determining its direction from the polar axis.*

Some people find the convention for ordered pairs in polar coordinates to
be unnatural. Though the first coordinate is *r*, it is sometimes
easier to think first of a point’s direction. Imagine a ray pointing in
the direction of θ, then imagine going *r* units in that direction
(backing up if *r* is negative). Thinking of a point’s direction
first also fits with the fact that θ often is taken as the independent
variable when displaying a graph in polar coordinates. You are free to
think of locating a point by any method you find most natural. Just
remember that the order of the coordinates is (*r*, θ) when
you represent a point in the polar plane.

Points in the polar plane do not have unique coordinates. Figure 3.9.5
shows a point in the polar plane located in two ways, both in the form (*r*, θ).
The point in Figure 3.9.5 can be located by the pair
(3.7 miles, 0.69 radians) and it can be located by the pair
(‑3.7 miles, 0.69‑π radians).

*Figure 3.9.5. Points in the polar plane
do not have unique coordinates.*

To imagine a graph in the polar plane involves the same ways of thinking
as imagining its graph in the rectangular plane. Imagine varying the
independent variable a little bit at a time while simultaneously keeping
track of the dependent variable’s value. Then record the covariation using
the conventions of your coordinate system. In most situations in which we
use polar coordinates, it is useful to treat θ as the independent variable
and *r* as the dependent variable.

Imagine two quantities whose values are represented by *r* and θ,
and that they have the relationship that *r* = 2θ – 5.
To represent this relationship in the polar plane, vary the value of θ,
keeping track of its value and the value of *r*. Recall that in
the polar plane, θ is a number of radians measured from the reference
direction and *r* is a number of unit lengths from the pole in the
direction of θ. Figure 3.9.6 shows points with coordinates (*r*, θ)
where *r* = 2θ ‑ 5. Be sure that you
understand what the coordinates of each ordered pair represent and why
these points are located where they are.

*Figure 3.9.6. Points for r = 2θ - 5
plotted in the polar plane. They appear jumbled, making it hard to see a
pattern in their generation.*

It is quite hard to envision the complete graph of *r* = 2θ - 5
in polar coordinates just from plotted points. The plotted points are too
jumbled to convey what happens as θ varies. A complete graph of *f*
in polar coordinates for 0 ≤ θ < 2π is given in Figure 3.9.7.

Imagine a ray, originating from the pole, being swept counter-clockwise as θ varies from 0 to 2π radians. At the same time that you sweep a ray from 0 to 2π radians, imagine the location at a distance of 2θ – 5 units in the direction that the ray points—going forward when 2θ – 5 is positive, and backward when 2θ – 5 is negative.

*Figure 3.9.7. Graph of $r=2\theta-5$ as
$\theta$ varies from 0 to $2\pi$.*

- For each of a-e, the coordinates on both sides of the equal sign are
in the polar system. Fill in the blank so that the polar coordinates on
the right locate the point represented by the polar coordinates on the
left. Remember that polar coordinates are in the form $(r,\theta)$.

- $(1, 2) = (-1,\_ \_)$
- $(-3, \sqrt 2) = (3, \_ \_)$
- $(3, -\pi/4) = ( \_ \_, 3\pi/4)$
- $(4.3, 27) = (-4.3, \_ \_)$
- $(-4.3, -27) = (4.3, \_ \_)$

- Download and print four copies of this polar coordinate paper and four copies of this second polar coordinate paper. Sketch a graph of each of a-d on both types of coordinate paper. Try not to plot points, but rather draw the graph smoothly while mentally estimating values of variables. Check your sketched graph with GC (type “theta” to get “θ”). Change the domains for r and θ in GC as needed.
- $r = 0.1\cdotθ,\text{ } 0 ≤ θ < 10π$
- $r = 5-θ, \text{ } 0 ≤ θ < 2π$
- $r = \mathrm{sin}(θ), \text{ } 0 ≤ θ < 2π$
- $θ = \mathrm{sin}(r), \text{ } -8 ≤ r ≤ 8$
- The animation below shows a machine gun rotating rapidly as it fires bullets. A bullet, once fired, travels at a constant speed in the direction it leaves the gun’s barrel.
- Why do the bullets make a spiral?
- Pause the movie. Do the bullets’ positions lie on an Archimedean spiral? Explain.
- Type the statement cos(0.8) into GC. It will display the value
0.69670671 (if you have GC set to display 8 significant digits). What
does this tell you about a point on the graph of
*y*= cos(*x*)? What does it tell you about a point on the graph of*r*= cos(θ)?

How far apart are two successive points that lie on the same ray from
the pole? (Graphs of *r* = *mθ*, *m* a
non-zero real number, are called Archimedean Spirals)

What are the coordinates of the point on the graph that is at the pole? Why does the graph never pass through (0, 0) even though it passes through the pole?

Why does this graph make a circle? What is its center and radius?

This graph is called the Batman Curve.

Explain how the variation in
*θ* relative to *r* over the following intervals
produces a piece of the displayed graph (It might help to print a
graph of *y* = sin(*x*) in Cartesian coordinates.):

-8 ≤ r ≤ -7

-7 ≤ r ≤ -6

-6 ≤ r ≤ -5

.
.
.

5 ≤ r ≤ 6

6 ≤ r ≤ 7

7 ≤ r ≤ 8