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Darren New wrote:
> Actually, I've never seen anyone get the answer write to "why are
> manholes round." I almost hope they ask me that at my next job
> interview, because the interviewer probably doesn't know either. ;-)
The snarky answer is "because manhole covers around round" is the
punchline of a joke, but does not answer the serious question at hand.
A manhole is *circular* minimize the amount of extra material needed to
make it impossible for the cover to fall into the manhole.
The manhole has a certain radius. The manhole cover has a slightly
higher radius. In every direction, the diameter of the cover is larger
than the diameter of the hole, taken in any direction. This is
accomplished if the cover's radius is even a millimeter larger, although
to allow for wear and the thermal expansion of the hole or contraction
of the cover, a larger difference--no more than a centimeter--is prudent.
With any other shape, the required amount to ensure non-falling-in
behavior is larger. A square, or a pentagon, or any n-gon, is larger
from corner to opposing corner (or most opposing corner for n-gons with
an odd number of vertices) than from side to opposing side (or corner to
opposing side). If the difference is larger than the extra amount added
on to make the cover bigger than the hole, the lid falls in, and so the
extra amount would need to be larger.
Minimizing the extra material is important because manhole covers are
heavy; adding to the radius greatly increases the weight of the cover,
because the weight is proportional to the area times the thickness.
Area is proportional to the square of the radius. As any programmer can
tell you, when faced with an O(n^2) problem that is unavoidable, you
make n as small as you can.
Also, a circular manhole fits in any direction. A polygonal manhole
cover has to be turned to fit, or else be picked up again (these things
are heavy) or the corners will wear (or break) off after a bunch of
near-misses.
Regards,
John
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