Why are Manhole Covers Round?

Have you ever noticed that every manhole cover you see is round? Have you ever wondered how engineers came to this decision? Did you realize that the reasoning comes down to the geometry of shapes?

That’s right: manhole covers are round because circles are the only shapes that cannot fall through themselves.

Let’s examine some of the properties of shapes to see why this is true.

First, it’s important to recognize that the shapes will only fall through themselves when they are rotated to be vertical. If they were lying flat then they would be covering the hole successfully!

Next, let’s examine shapes with unequal sides beginning with a triangle. If the triangle had side lengths of 3, 3, and 2 feet then we could rotate it so that the side with length 2 feet was parallel to the ground. This would mean that the width of the manhole cover would now be 2 feet, with the width of the opening approaching 3 along the sides of the hole. Since 3 is greater than 2, the cover would be able to fall through the opening. Similarly for a 4-sided figure if one side is shorter or longer than the others then we will find the same result.

There is a pattern to notice from examining the previous shapes. The shortest width of each shape is compared to the longest length. But what if the covers were regular shapes with all sides and angles being the same?

Let’s look at a square with length of 1 foot on each . The shortest width of a square will be the length of a side. But how about the longest length? That can be found by measuring from the upper left-hand corner to the lower-right hand corner.

Using Pythagorean’s Theorem, we know that 1^2 + 1^2 = diagonal^2. So the diagonal is square root of 2, or approximately 1.414. This means that this square could still fall through itself.

So now our focus can shift to finding a shape whose longest length and shortest width are the same. But this would mean that we would need all widths and lengths to be the same because we cannot have the longest length be shorter than the shortest width. This leads us to a circle.

A circle will always have the width of its diameter no matter which way it is rotated, so this will be the shortest width. But the longest length will also be the diameter as well because any chord will be shorter than the diameter. So the circle is unique to all polygons and shapes in that it can never fall through itself.

Want to test geometry with physical objects? Since manhole covers are heavy you should explore this idea using Tupperware containers. See how many different lid shapes will fall into the container, and how many will not – then post your comments!


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We want and need your ideas, suggestions, and observations. What would you like to know more about? What questions have your students asked? We invite you to share with us and other readers by posting your comments. Please check back often for our newest posts, subscribe via email, or download the RSS feed for this blog. Let us know what you think and tell us how we can serve you better. We appreciate your feedback on all of our Middle School Portal 2 publications. You can also email us at msp@msteacher.org.


This post was originally posted in the Everyday Explanations – Answers to Questions Posed on a Middle School Bus Ride by Sean Mittleman. We have his permission to re-post in the MSP2 blog.

Think Globally and Locally, Mathematically

Student Explorations in Mathematics, formerly known as Student Math Notes, is an official publication of the National Council of Teachers of Mathematics (NCTM) and is intended as a resource for grades 5-10 students, teachers, and teacher educators. Each issue develops a single mathematical theme or concept in such a way that fifth grade students can understand the first one or two pages and high school students will be challenged by the last page. The content and style of the notes are intended to interest students in the power and beauty of mathematics and to introduce teachers to some of the challenging areas of mathematics that are within the reach of their students.

The teacher version includes additional information on world poverty as well as instructional ideas to facilitate classroom discourse. The student guides are available for free download (see below) but the teacher’s guides are only available with NCTM membership.

In the following activities from the May 2011 and September 2011 issues of the magazine, students use histograms and make comparisons between different country groups, then create graphs that compare these differences in many ways and consider how each of these displays might be used. In part 2, students consider important information about world poverty by using measures of central tendency and box plots. Students analyze data and use a hands-on manipulative to interpret and understand box plots, including the connection between percentiles and quartiles.

Part 1: Hunger at Home and Abroad (May 2011)
World Poverty Data can be downloaded here.

Part 2: Poverty at Home and Abroad (September 2011)
World Poverty Data can be downloaded here.


We Want Your Feedback
We want and need your ideas, suggestions, and observations. What would you like to know more about? What questions have your students asked? We invite you to share with us and other readers by posting your comments. Please check back often for our newest posts or download the RSS feed for this blog. Let us know what you think and tell us how we can serve you better. We appreciate your feedback on all of our Middle School Portal 2 publications. You can also email us at msp@msteacher.org.

What Can Batting Averages Tell Us?

It’s the bottom of the 9th, 2 outs, bases loaded in the 7th game of the World Series. On the mound is the opposing team’s left-handed pitcher trying to close out the game. As the Head Coach you have a decision to make: let your left-handed 9th batter hitting .270 for the season go up and take his hacks, or pinch hit with your young, recently called-up rookie batting .350?

The first question we need to answer before making a decision is: What do the batting average numbers mean?

Batting averages are a simple decimal that approximates the number of hits per at-bat, or more simply the probability that a batter reached first base on a hit during his previous at-bats. The equation used to calculate batting average is simple: # Hits/# At-Bats.

A batting average is written in decimal form using 3 digits after the decimal point. Avid baseball readers read these as large numbers, so .400 would be read as “four-hundred” and .283 would be read as “two eighty-three.” Each individual thousandth is called a “point,” so .400 would be considered 117 points higher than .283.

But not all batting averages can be read equally. Two players can have the same batting average, take .300 for example, and have very different statistics. Player 1 could have 3 hits in 10 at-bats while player 2 may have 120 hits in 400 at-bats.

So which is a more accurate description of a player’s ability? Let’s take a look at what happens to the players after their next at-bat.

If they were to both get a hit in the next at-bat, their averages would indicate that Player 1 is much more likely to get a hit, yet if they both made an out the numbers would swing heavily in favor of Player 2.

The key to this discrepancy lies in the number of total at-bats. With more at-bats, the denominator for the fraction becomes larger and is less affected by adding 0 or 1 to the numerator. Referring to the chart, the next at-bat for Player 1 will either increase his average by 64 points or decrease it by 27. Player 2 will see either a 2 point increase or a 1 point decrease. So batting averages are less affected with larger numbers of at-bats, and can more accurately describe a hitter’s tendency over a period of time.

Now, looking back to the original question, I will add more context to the problem. In an average 162-game season a player might amass about 450 at-bats, and back-ups could see 100 at-bats. Rookies and recent call-ups (players invited to the major-league team from the minor leagues) will usually be on the team for the final 50 games of the season.

Knowing this information and having seen the chart from above, does this change your original decision for what to do? Why or why not? There is no definitive correct answer to this question, but I do ask that you use numbers to support your reasoning. Please post your decisions in the comments.


We Want Your Feedback
We want and need your ideas, suggestions, and observations. What would you like to know more about? What questions have your students asked? We invite you to share with us and other readers by posting your comments. Please check back often for our newest posts or download the RSS feed for this blog. Let us know what you think and tell us how we can serve you better. We appreciate your feedback on all of our Middle School Portal 2 publications. You can also email us at msp@msteacher.org. Post updated 11/27/2011.


This post was originally posted in the Everyday Explanations – Answers to Questions Posed on a Middle School Bus Ride by Sean Mittleman. We have his permission to re-post in the MSP2 blog.

Free Articles from NCTM Middle School Math Journal

Take a free look at the articles from the NCTM middle school journal – Mathematics: Teaching in the Middle School. Explore and share with others. Check out the Reflection Guides. They provide professional development support linked to specific articles for individual, small groups or school use.Digesting

Student-Authored Story Problems
August 2010, Volume 16, Issue  1

Problems with nth-Term Problems:
Reflect and Discuss
September 2010, Volume  16, Issue  2

Map the Race to the White House
October 2010, Volume  16, Issue  3

Exploring Our Complex Math Identities
November 2010, Volume  16, Issue  4

Multiplication Fact Fluency Using Doubles
December 2010, Volume  16, Issue  5

Fold in Origami and Unfold Math
February 2011, Volume  16, Issue  6

All These Rays! What’s the Point?
March 2011, Volume  16, Issue  7

Problem Solving around the Corner
April 2011, Volume  16, Issue  8

Tailoring Tasks to Meet Students’ Needs
May 2011, Volume  16, Issue  9

Article and Reflection Guide – Technology
Technology Enhances Student Learning across the Curriculum
Several forms of technology that can enhance basic understandings and skills of middle school mathematics concepts are discussed, including technologies such as Calculator Based Ranger (CBR), Spreadsheets, and Geometer’s Sketchpad. February 2004, Volume 9, Issue 6, pp. 344-349

Happy Belated Pi Day!

I’m sure lots of folks celebrated Pi Day on 3/14 – this post is for those folks that sorta forgot! Anyway – these great ideas, lessons, and activities can be used throughout the year. Please share your favorite Pi lessons in the comment area.

Ways to Think About Pi – from the Ohio Resource Center for Math, Science, and Reading – includes facts about Pi and lessons and activities

Going Around In Circles – from the Middle School Portal 2 (MSP2) project

These next three resources come from Wired Science and include background information and lesson ideas:

How Do You Determine Pi Without a Circle?

Activities for Pi Day

Pi: How Many Digits Do You Need?

You can also do a search in the Middle School Portal 2 Digital Library – you’ll get back resources that right on track for middle school math students.

We Want Your Feedback
We want and need your ideas, suggestions, and observations. What would you like to know more about? What questions have your students asked? We invite you to share with us and other readers by posting your comments. Please check back often for our newest posts or download the RSS feed for this blog. Let us know what you think and tell us how we can serve you better. We appreciate your feedback on all of our Middle School Portal 2 publications. You can also email us at msp@msteacher.org. Post updated 4/05/2012.