Slicing Pizzas, Racing Turtles, and Further Adventures in Applied Mathematics (Princeton Puzzlers) - Softcover

Banks, Robert B.

Softcover

ISBN 10: 0691154996 ISBN 13: 9780691154992

Publisher: Princeton University Press, 2012

Synopsis

Have you ever daydreamed about digging a hole to the other side of the world? Robert Banks not only entertains such ideas but, better yet, he supplies the mathematical know-how to turn fantasies into problem-solving adventures. In this sequel to the popular Towing Icebergs, Falling Dominoes (Princeton, 1998), Banks presents another collection of puzzles for readers interested in sharpening their thinking and mathematical skills. The problems range from the wondrous to the eminently practical. In one chapter, the author helps us determine the total number of people who have lived on earth; in another, he shows how an understanding of mathematical curves can help a thrifty lover, armed with construction paper and scissors, keep expenses down on Valentine's Day.


In twenty-six chapters, Banks chooses topics that are fairly easy to analyze using relatively simple mathematics. The phenomena he describes are ones that we encounter in our daily lives or can visualize without much trouble. For example, how do you get the most pizza slices with the least number of cuts? To go from point A to point B in a downpour of rain, should you walk slowly, jog moderately, or run as fast as possible to get least wet? What is the length of the seam on a baseball? If all the ice in the world melted, what would happen to Florida, the Mississippi River, and Niagara Falls? Why do snowflakes have six sides?


Covering a broad range of fields, from geography and environmental studies to map- and flag-making, Banks uses basic algebra and geometry to solve problems. If famous scientists have also pondered these questions, the author shares the historical details with the reader. Designed to entertain and to stimulate thinking, this book can be read for sheer personal enjoyment.

About this title

About the Author

Robert B. Banks (1922-2002) was the author of Towing Icebergs, Falling Dominoes, and Other Adventures in Applied Mathematics (Princeton). He was professor of engineering at Northwestern University and dean of engineering at the University of Illinois at Chicago.

Excerpt. © Reprinted by permission. All rights reserved.

Slicing Pizzas, Racing Turtles, and Further Adventures in Applied Mathematics

By ROBERT B. BANKS

Princeton University Press

Copyright © 1999 Princeton University Press
All right reserved.
ISBN: 978-0-691-15499-2

Contents

Preface...............................................................................ixAcknowledgments.......................................................................xiiiChapter 1 Broad Stripes and Bright Stars..............................................3Chapter 2 More Stars, Honeycombs, and Snowflakes......................................13Chapter 3 Slicing Things Like Pizzas and Watermelons..................................23Chapter 4 Raindrops Keep Falling on My Head and Other Goodies.........................34Chapter 5 Raindrops and Other Goodies Revisited.......................................44Chapter 6 Which Major Rivers Flow Uphill?.............................................49Chapter 7 A Brief Look at π, e, and Some Other Famous Numbers......................57Chapter 8 Another Look at Some Famous Numbers.........................................69Chapter 9 Great Number Sequences: Prime, Fibonacci, and Hailstone.....................78Chapter 10 A Fast Way to Escape.......................................................97Chapter 11 How to Get Anywhere in About Forty-Two Minutes.............................105Chapter 12 How Fast Should You Run in the Rain?.......................................114Chapter 13 Great Turtle Races: Pursuit Curves.........................................123Chapter 14 More Great Turtle Races: Logarithmic Spirals...............................131Chapter 15 How Many People Have Ever Lived?...........................................138Chapter 16 The Great Explosion of 2023................................................146Chapter 17 How to Make Fairly Nice Valentines.........................................153Chapter 18 Somewhere Over the Rainbow.................................................163Chapter 19 Making Mathematical Mountains..............................................177Chapter 20 How to Make Mountains out of Molehills.....................................184Chapter 21 Moving Continents from Here to There.......................................196Chapter 22 Cartography: How to Flatten Spheres........................................204Chapter 23 Growth and Spreading and Mathematical Analogies............................219Chapter 24 How Long Is the Seam on a Baseball?........................................232Chapter 25 Baseball Seams, Pipe Connections, and World Travels........................247Chapter 26 Lengths, Areas, and Volumes of All Kinds of Shapes.........................256References............................................................................279Index.................................................................................285

Chapter One

Broad Stripes and Bright Stars

These days we see much more of the flag of the United States than we ever did in the past. Old Glory flies over many more office buildings and business establishments than it did before. It is now seen far more extensively in parks and along streets and indeed in a great many programs and commercials on television.

With this greatly increased presence and awareness of the flag, there is understandable growing interest in learning more about numerous aspects of the U.S. flag, including its history and physical features and the customs and protocol associated with it.

There are a number of books that examine various topics concerning the flag of the United States. Representative are the publications of Smith (1975a) and Furlong and McCandless (1981). On the world scene, many references are available dealing with the flags of all nations. For example, the books by Smith (1975b) and Crampton 1990 cover many subjects relating to flags of the world and to various other topics of vexillology: the art and science of flag study.

The Geometry of the Flag of the United States

With that brief introduction, we come directly to the point. The colors of the U.S. flag are red, white, and blue. Now, are you ready for the big question? What are the area percentages of red, white, and blue? That is, which of the three colors occupies the largest area of the flag and which color the smallest? It's a good question. Do you want to guess before we compute the answer?

The flag is shown in figure 1.1 and its more important proportions and features are listed in table 1.1. Arbitrarily selecting the foot as the unit of linear measurement, here are some preliminary observations:

The total area of the flag is 1.0 × 1.9s1.9 ft² The area of the union is 7 13 × 0.76 0.4092 ft² The length of the seven upper stripes is 1.14 ft The length of the six lower stripes is 1.9 ft

The width of a stripe is 1 13 0.07692 ft

The problem of computing the red area is easy

The problem of computing the white and blue areas is not so easy because of the 50 white stars in an otherwise blue union

So, before we can obtain the final answer we have to look at stars

The Geometry of a Five-Pointed Star

A five-pointed star, commonly called a pentagram, is shown in figure 1.2(a). Its radius, R, is the radius of the circumscribing circle. The five-sided polygon within the star is called a pentagon; the radius of its circumscribing circle is r.

The section ABOC is removed from the pentagram of figure 1.2(a) and displayed as the pentagram kite of figure 1.2(b). Some geometry establishes that α 36°, ß 72°, and γ 126°. Without much difficulty we obtain the expression

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1.1)

where A is the area of the pentagram. Substituting the values of a and ß into this expression gives

A 1.12257R². (1.2)

For comparison, remember that the area of the circumscribing circle is πR² where, of course, π 3.14159. Equation 1.2 provides us with a simple formula to compute the area of a five-pointed star.

In elementary mathematical analysis we frequently run across the numerical quantity φ (1 2)(1 [square root of 5]) 1.61803. It is a very famous number in mathematics. It is called the golden number or divine proportion. Our pentagram is full of golden numbers. According to Huntley (1970), the following φ relationships prevail in figure 1.2 based on unit length BC i.e., one side of the pentagon:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.3)

Utilizing these relationships, it can be established that the area of the regular pentagram can be expressed in terms of φ:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.4)

As we would expect from observing equation (1.2), the quantity in the brackets of equation (1.4) is 1.12257.

The perimeter of a pentagram is not difficult to determine. Using some geometry and trigonometry we obtain

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.5)

The bracketed quantity has the numerical value 7.2654. The length of the circumscribing circle is, of course, 2 πR.

Numerous other relationships could be established. For example, can you demonstrate that the ratio of the area of the five points of the pentagram to the area of the base pentagon is [square root of 5]?

How Much Red, How Much White, How Much Blue?

We now have the information we need to answer the big question. From table 1.1 we note that the radius of a star is R 0.0308 ft and so, from equation (1.2), the area of a single star is A 0.0010649 ft². The area of 50 stars is 0.05325 ft².

Color: red

The red area, A_r, is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Color: white

The white area, A_w, is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Color: blue

The blue area, A_b, is

A_b (0.76 × 7 13) 0.05325 0.35598 ft².

The total area of the flag is A_flag 1.9 ft². So it is easy to flag calculate that the color percentages are 41.54% red, 39.73% white, and 18.73% blue. This means that if you are going to paint a really big U.S. flag, you will need 42 gallons of red paint, 40 gallons of white, and 19 gallons of blue to come out about even.

Here are several other items of information about the flag that you might want to confirm:

Based on a flag width of 1.0 foot, there are 10.26 feet of red stripe and 9.12 feet of white stripe. The total is 19.38 feet.

The union is 13.01% white and 86.99% blue.

If the 50 stars were replaced by a single big star with the same total area, it would have a radius of 0.218 feet.

The perimeter of a single star is 0.2238 feet; the total perimeter of all 50 is 11.19 feet. This is nearly double the perimeter of the entire flag.

This is sufficient. We stop here because we now have the information we set out to determine. That is, the red area is 41.6%, the white 39.7%, and the blue 18.7%.

Main Dimensions of Flags

The official flag of the United States has a ratio of width to length of 10:19. Why the relative length of the flag is precisely 1.9—or indeed why the relative length of the union is exactly 0.76 (could it be 1776?)—is not known. It just is. If you are interested in the historical aspects of vexillology, you might want to contact the Flag Research Center in Winchester, Massachusetts or the Flag Institute in Chester, England.

In any event, an interesting question has been raised by Nicolls (1987): "What are the ideal proportions of a flag?" Again, this is the kind of question for which there is no "scientific" answer. A brief history of the changes of flag dimension proportions is given by Nicolls. During the Middle Ages, for example, flag proportions ranged from an extremely short 1(width):0.5(length) ratio to a square 1:1 ratio.

Over the years, the poor visibility and inadequate flapping characteristics of quite short flags resulted in their gradual lengthening. At present, the official flags of the United Kingdom and thirty other nations have proportions of 1:2 and those of twenty five other countries possess ratios of 3:5. It is pointed out by Nicolls that these ratios are numerically close to that given by the golden mean, 1:φ, where φ (1 2)(1 [square root of 5]) 1.618. We see that collectively the geometric proportions of the world's flags are not greatly different from the "divine proportion."

A word about flapping or fluttering of flags. This is an interesting and not very easy problem in fluid mechanics. If you are challenged by the mathematics of the phenomenon you can study the section on "surface waves" in Lamb (1945). If you are interested in some experimental work, get yourself an electric fan, a few sheets of paper, a pair of scissors, and go to work.

PROBLEM 1. Now that you know all about the geometrical features of the flag of the United States, your assignment is to select and analyze the flag of some other country. As in the case of the U.S. flag, you are required to determine the percentage distribution of the colors of the flag you choose. A list of flags appears in table 1.2. Since some flags are geometrically simple and others are rather complicated, there is considerable spread in the number of points you will be awarded.

PROBLEM 2. If the width to length ratio of the U.S. flag were altered from 1:1.9 to 1: φ, and it was otherwise unchanged, what would be the area percentages of red, white, and blue?

Answer. Red: 39.4%, white: 38.6%, blue: 22.0%.

Revisiting the Stars

The pentagram—the five-pointed star—has a very long history. Its geometrical properties were known to the ancient Babylonians and it was regarded as a symbol and badge of the Grecian school of geometers in the days of Pythagoras.

We have seen the many relationships between the pentagram and the golden number, φ. Furthermore, as we shall learn later on, φ is the ratio of successive Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, ... when these numbers become very large. So we note the connection between pentagrams and this famous mathematical sequence.

There are many interesting and practical problems involving pentagrams and pentagons; some are easy and some are difficult. For example, the area of the pentagon portion of a pentagram is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1.6)

where r is the radius of the circumscribing circle of the pentagon. In addition, the perimeter of a regular pentagon is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.7)

For example, the famous Pentagon building near Washington has a side length L 921 ft. Hence, the total perimeter is 4,605 feet. From equation 1.7 the radius r 783 ft and so, from equation 1.6 , the total area covered by the Pentagon is A 1,457,680 ft². This is approximately 33.5 acres. Some questions: If they ever add the five points to the Pentagon building thereby making it the Pentagram building what will be the radius R, the perimeter P, and the total area A? The answers are that R 2,050 ft, P s14,895 ft, and A 108.3 acres.

The Ubiquity of φ: Three Problems

In the preceding sections we have seen that the golden ratio, φ (1 2)(1 [square root of 2]) 1.61803, makes numerous appearances in the patterns of pentagrams and pentagons. In later chapters, we take closer looks at this very interesting number. For the present, here are three problems that illustrate that φ shows up not only in the geometry of five-pointed stars but in many other places as well.

GOLDEN EARRINGS. A circle of radius r is removed from a larger circle of radius R, as shown in figure 1.3. The center of gravity of the remaining area is at the edge of the removed circle, P. Confirm the result obtained by Glaister 1996 that this "golden earring" indeed balances at point P, if the ratio of the radii of the two circles is R/r = φ.

PENTAGON CIRCLES. An interesting design or pattern for a flag might be the array of circles shown in figure 1.4. In this figure, five circles, each of radius r, are placed with their centers at the corners of a regular pentagon and their circumferences passing through the centroid of the pentagon, O.

Show that the radius R of the largest circle that can be covered by the five smaller circles is given by R/r = φ.

This problem was originally posed by the noted English mathematician E. H. Neville in 1915; it is discussed by Huntley (1970)

FOLDED PENTAGRAM. In figure 1.2, triangle ABC is folded about BC and the other four triangles are folded in the same fashion. The five sloping triangular sides meet at a point to create a "folded pentagram" pyramid with a pentagon base of radius r.

Show that the height of the pyramid is H/r = φ and its volume is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (1.8)

Chapter Two

More Stars, Honeycombs, and Snowflakes

Skinny Pentagrams and Fat Pentagrams

So far, in our study of pentagrams—that is, five-pointed stars—we have considered only the quite familiar pentagram that appears on the flag of the United States and in so many other places. We shall say that the five points that appear on this particular pentagram are normal or "regular" in shape.

In contrast, our next step is to examine the geometry of pentagrams with points that range in shape from "long and skinny" to "short and fat." Let figure 2.1 serve as our definition sketch.

The main geometrical variable in our problem is the ratio r R, in which R is the radius of the external circumscribing circle and r is the radius of the internal circumscribing circle. It is clear that the angle ß 72° regardless of the value of r R (because 5 × 72° 360°). For small values of r R, we have skinny pentagrams and for large values we have fat ones. In between, if r R 1 φ², where φ (1 2)(1 [square root of 5]), we have our familiar regular pentagram. These three cases are illustrated in figure 2.2.

It is logical to identify the regular pentagram as that pentagram for which α 36° or, equivalently, r R 1 φ². An array of adjectives is available to define the cases for which α 36° (long and skinny) and alpha] 36° (short and fat). The terminology acute–regular–obtuse seems to be the simplest and most descriptive way to define the three categories of pentagrams.

Some Geometrical Features of Pentagrams

Utilizing figure 2.1 and the law of sines, we obtain the equations

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2.1)

We want to express the various quantities in our pentagram problem in terms of the variable r R. Letting p r R and employing the above equations, we determine that the area of the pentagram is

A R² 5p sin(ß 2), (2.2)

and the perimeter is

P R 10p [sin(ß 2)/sin(α 2)]. (2.3)

The results of computations based on these equations are listed in table 2.1. We make the following observations:

Clearly, if p 1 φ² 0.38197, the pentagram becomes the familiar five-pointed star.

When p 0.5, the pentagram takes on the shape of the star used in the Berghaus projection of the world.

When p 1 φ 0.61803, both α and ß are 72° and the pentagram is composed of ten identical triangles.

When p φ 2 0.80902, the pentagram becomes a regular pentagon. As we observe in the table, the perimeter is a minimum for this value of p.

When p 1, the pentagram becomes a regular decagon (i.e., a ten-pointed polygon).

Berghaus Star Projection of the World

When r R 0.5, the radius of the internal polygon is one-half the radius of the pentagram. As mentioned earlier, this is the radius ratio of the Berghaus star projection of the world.

Later on, in chapter 22 on cartography, this projection is examined again. For now, we simply make the observation that in the Berghaus projection, the northern hemisphere of the world is displayed in the internal circumscribing circle of the star. The southern hemisphere appears in the five points of the star defined at 728 intervals along the equator. A recommended reference on this topic is Snyder (1993).

Six-Pointed Stars: Hexagrams and Hexagons

We conclude our study of the geometry of stars with a brief analysis of six-pointed stars or hexagrams. A definition sketch appears in figure 2.3. The radius of the hexagram is R. The six-sided polygon inside the hexagram is called a hexagon; its radius is r.

In comparison with the geometry of a five-pointed star—the pentagram—that of the six-pointed star is simple. This is because the hexagram and its associated hexagon are composed of nothing but equilateral triangles. In figure 2.3, ABC is a representative triangle. Since the length of each of its three sides is r and each of the internal angles is 60°, it is not difficult to establish that the area of each triangle is ([square root of 3] 4)r² and the perimeter of each is 3r.

With this information, we easily determine that the area and perimeter of a hexagon are

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2.4)

Likewise, the area and perimeter of a hexagram are

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (2.5)

Finally, we note that R [square root of 3]r and hence the radius ratio is p r R 1 [square root of 3] 0.577. The area and perimeter of the hexagram can be expressed in terms of radius R instead of r. The answers are

A [square root of 3] R²; P 4[square root of 3] R. (2.6)

PROBLEM In the previous chapter, we determined the area percentages by color of the flag of the United States. It is observed that the Star of David, which appears on the flag of Israel, is a regular hexagram. What are the area percentages by color of this flag?

Honeycombs and Hexagons

Leaving the subjects of flags and stars, we proceed to an entirely different topic. Suppose we want to store a certain commodity, for example, corn, in a structure whose shape will provide the maximum volume of storage for a specified length of storage boundary. What shape should be used? The answer: a circle. That's why circular silos and elevators are used for the storage of corn, wheat, and other grains. It is easy to establish that the ratio of the area of a circle to its perimeter is A P π r² r 2 0.500r.

(Continues...)

Excerpted from Slicing Pizzas, Racing Turtles, and Further Adventures in Applied Mathematicsby ROBERT B. BANKS Copyright © 1999 by Princeton University Press. Excerpted by permission of Princeton University Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Princeton University Press
• Publication date: 2012
• Language: English
• ISBN 10: 0691154996
• ISBN 13: 9780691154992
• Binding: Paperback
• Number of pages: 302