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/