If you Google "why are manhole covers round" you get 23,500 hits. I did
not look at all of them, but the ones I looked at tend to fall into
three groups; the third of which is an extension of the second.
(1) some logical reasons why round is a good idea
(2) that an answer is not really desired, it is really an interview
question to see how the respondent thinks
(3) legends/jokes about how certain people (like Feynman) would have
responded to the question
I think there are indeed some good reasons for choosing round.
Therefore, it is not clear to me that the original connection between
this and the "F causes a" is a good connection. Perhaps John can
explain more about his intent.
For those who would rather not read a bunch of Google hits, here are
some reasons for round that I found. I think each has merit.
(1) The round cover won't fall in. (Note this doesn't have to be about
safety for people underneath, since you wouldn't normally replace the
cover with someone in the hole. Think of what a pain it would be if a
heavy rectangular lid would fall in, and you would have to retrieve it.
That would be very dangerous to retrieve manually, and may require
OSHA-approved equipment procedures to so it safely. That incurs a lot
of time and cost. You really don't want a cover to fall in, even if no
one is below.)
(2) Once removed, it is easy to roll out of the way, and also easy to
roll back into place.
(3) It doesn't have to be aligned with anything when put back in place.
(4) It is easier to manufacture both in terms of casting and fitting.
(5) The round and frame are less likely to be deformed by external
pressures, making it less likely the cover could get wedged in place and
that it will go back into place.
(6) Since most covers are cast iron, and cast iron can break/crack, it
is nice to have rounded edges rather than corners that can break off.
If corners bend some rather than break, then the cover doesn't seat
properly, and if they bend up the corners can be hard on tires.
(7) [This one is less obvious to me, but it appears on the website of a
manhole cover foundry] The round cover stays in place better as traffic
drives over it.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu