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### Why are Manhole Covers Round?

```
Date: 05/09/2000 at 15:16:45
From: Cira Parisi
Subject: Geometry

Why are most manhole covers round? Why aren't manhole covers on the
streets squares (or rectangles)? I've tried searching on the Internet
for the answer. I know that the shapes of many objects relate directly
to their function.
```

```
Date: 05/09/2000 at 15:41:22
From: Doctor Douglas
Subject: Re: Geometry

Hi Cira,

Thanks for sending your question to Ask Dr. Math. You are absolutely
correct in saying that the shapes of many objects relate directly to
their function. After all, it would not do to have wheels that were
not round, right? (But see some of the Web sites below.)

The standard answer to your question is that the circular shape
possesses the nice property that it is impossible for the thin cover
to drop through the hole (and possibly injure someone below). A square
or rectangular cover could accidentally pass through the hole along
the diagonal of the hole.

For a site that address the manhole cover question directly, see
"Manhole Covers" by Stephen E. Collins at:

http://web66.umn.edu/Ramble/manhole/manhole.html

And see "Rolling with Reuleaux," by Ivars Peterson:

http://www.maa.org/mathland/mathland_10_21.html

Hope that helps,

- Doctor Douglas, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Conic Sections/Circles
High School Geometry
High School Triangles and Other Polygons
Middle School Conic Sections/Circles
Middle School Geometry
Middle School Triangles and Other Polygons

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