Polar Coordinates

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Polar Coordinates

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One of several systems for addressing points in the plane is the polar-coordinate system. In this system a point P is identified with an ordered pair (r, θ) where r is a distance and θ an angle. The angle is measured counterclockwise from a fixed ray OA called the polar axis. The distance to P is measured from the end point O of the ray. This point is called the pole. Thus each pair determines the location of a point precisely (Figure 1).

When a point P is given coordinates by this scheme, both r and θ will be positive. In working with polar coordinates, however, it occasionally happens that r, θ, or both take on negative values. To handle this one can either convert the negative values to positive by appropriate rules, or one can broaden the system to allow such possibilities. To do the latter,

instead of a ray through O and P one can imagine a number line with θ the angle formed by OA and the positive end of the number line, as shown in Figure 2. one can also say that an angle measured in a clockwise direction is negative. For example, the point (5, 30°) could also be represented by (5, 150°).

To convert r and θ to positive values, one can use these rules:

I (r, θ) = (r, θ ± π) or (r, θ± 180°)

II (r, θ ± 2π) or (r, θ ± 360°)

(Notice that θ can be measured in radians, degrees, or any other measure as long as one does it consistently.) Thus one can convert (5, 150°) to (5, 30°) by rule I alone. To convert (7, 200°) would require two steps. Rule I would take it to (7, 20°). Rule II would convert it to (7, 340°).

Rule II can also be used to reduce or increase θ by any multiple of 2π or 360°. The point (6.5, 600°) is the same as (6.5, 240°), (6.5, 960°), (6.5, 120°), or countless others.

It often happens that one wants to convert polar coordinates to rectangular coordinates, or vice versa. Here one assumes that the polar axis coincides with the positive x-axis and the same scale is used for both (Figure 3). the equations for doing this are

θ arc tan y/x

x= r cos θ

y = r sin θ

For example, the point (3, 3) in rectangular coordinates becomes (, 45°) in polar coordinates. The polar point (7, 30°) becomes (6.0622, 3.5). Some scientific calculators have built-in functions for making these conversions.

These formulas can also be used to convert equations from one form to the other. The equation r= 10 is the polar equation of a circle with it center at the origin and a radius of 10. Substituting for r and simplifying the result gives x2+ y2= 100. Similarly, 3x 2y= 7 is the equation of a line in rectangular coordinates. Substituting and simplifying gives r= 7/(3 cos θ 2 sin θ) as its polar equation.

As these examples show, the two systems differ in the ease with which they describe various curves. The Archimedean spiral r= kθ is described simply with polar coordinates. In rectangular coordinates, it is a mess. The parabola y= x2 is simple. In polar form it is r= sin θ/(1 sin2 θ). (This comparison is a little unfairthe polar forms of the conic sections are simpler if one puts the focus at the pole.)

One particularly interesting way in which polar coordinates are used is in the design of radar systems. In such systems, a rotating antenna sends out a pulsed radio beam. If that beam strikes a reflective object the antenna will pick up the reflection. By measuring the time it takes for the reflection to return, the system can compute how far away the reflective object is. The system, therefore, has the two pieces of information it needs in order determine the position of the object. It has the angular position, θ, of the antenna, and the distance r, which it has measured. It has the objects position (r, θ) in polar coordinates.

KEY TERMS

Coordinate system A system of describing the position of points by means of combinations of numbers.

Polar coordinates A coordinate system in which one number represents a distance and the other number an angle.

Rectangular coordinates A coordinate system in which two or more numbers represent distances in mutualy orthogonal directions.

For coordinating points in space a system known as cylindrical coordinates can be used. In this system, the first two coordinates are polar and the third is rectangular, representing the points distance above or below the polar plane. Another system, called a spherical coordinate system, uses a radius and two angles, analogous to the latitude and longitude of points on earth.

Polar coordinates were first used by Isaac Newton and Jacob (Jacques) Bernoulli in the seventeenth century, and have been used ever since. Although they are notas widely used as rectangular coordinates, they are important enough that nearly every book on calculus or analytic geometry will include sections on them and their use; and makers of professional quality graph paper will supply paper printed with polar-coordinate grids.

Resources

BOOKS

Ball, W.W. Rouse. A Short Account of the History of Mathematics. London: Sterling Publications, 2002.

Finney, Ross L., et al. Calculus: Graphical, Numerical, Algebraic, of a Single Variable. Reading, MA: Addison Wesley Publishing Co., 1994.

Drexel University The Math Forum. Ask Dr. Math FAQ: Polar Coordinates. <http://mathforum.org/dr.math/faq/formulas/faq.polar.html> (accessed October 8, 2006).

J. Paul Moulton

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Polar Coordinates

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