Why are Manholes Round?
Date: 8/20/95 From: Anonymous Subject: Man hole My daughter and I have spent an entire day trying to locate the answer to the following question. The public librarian could not help us locate the answer. Why is a manhole round? We think it has something to do with physics. Thank you.
Date: 8/26/95 at 17:3:59 From: Dr. Ken Subject: Re: Man hole Hello there! I'd say it has more to do with Geometry than Physics. It's because a circle is the only shape that won't fall through its own hole. For instance, if it were a square, you could drop it through diagonally, because the diagonal "diameter" of a square is longer than the "diameter" striaght across. For any polygon, there will be "diameters" of different lengths, allowing you to turn the cover so that the shortest diameter of the cover lines up with the longest diameter of the hole, and it could fall through. Dr. Ken, The Math Forum -Check out our web site http://mathforum.org/dr.math/
Date: Sat, 7 Jun 1997 From: Robert Vaul Subject: Round Manhole Covers Physics is a partial answer; another reason is that the shape causes less damage compared to shapes with corners. - Robert Vaul
Date: Sun, 8 Jun 1997 From: Dr. Math Subject: Round Manhole Covers Hello Robert - Here's an article by Ivars Peterson about manhole covers and curves of constant widths: http://www.maa.org/mathland/mathland_10_21.html "Rolling with Reuleaux" - Ivars Peterson (MathLand) Why is the cover of a manhole round? The usual answer is that a circular lid, unlike a square or hexagonal cover, won't fall through the opening. The circle works because it has a constant width, defined as the distance between a pair of parallel lines touching the curve on opposite sides. For a circle, the width is simply the circle's diameter. However, the circle isn't the only curve of constant width. There is actually an infinite number of such curves, any one of which could form a manhole lid... The simplest such curve is known as the Reuleaux triangle... It's possible to construct a curve of constant width not only from an equilateral triangle but also from any polygon with an odd number of sides.... Your observation of course is still relevant. Dr. Sarah, The Math Forum -Check out our web site http://mathforum.org/dr.math/
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