I've never thought about it, but a circle of the correct size has no less reason to not fall in than say a square or triangle of the appropriate size. I.e. if it's smaller than the hole, it'll most likely fall in.
My theory: They're heavy, so making it a circle at least eliminates the need to "align" it when putting it back. You can basically just drag it using a hook and it'll slot itself into place.
Equilateral triangles can fall in. You have three dimensions: the height of the triangle is less than the side length of the triangle, so it fits in near the sides of the hole.
Which includes circles and Reuleaux triangles, which are much more difficult to manufacture than circles. I think this can be rounded off to "circles are the only well-known shape that can't fall into a similarly-sized hole".
I think the point is that a circle is not the only shape that can't fall into it's own hole/fitting, so it's the combination of this fact along with other factors that make a circular cover popular.
But manhole covers are always bigger than the hole. Otherwise, they'd fall in at installation and the problem would be immediately obvious.
The point here is more that a circle has the same diameter regardless of orientation. There's no way to rotate it to make it fit through a hole it couldn't before, like you can do with a rectangle.
Actually, yes, that's exactly what I was getting at. You'd have to make the square cover sqrt(2) times wider than the hole, but you could still prevent it from ever falling in. (Granted, it's not a very efficient solution, though.)
Yes, this is how I usually do it and it's not uncommon that it becomes an issue when moving the circle.
To be fair there is usually a fillet or something connected to the hole, but I still don't see why that should be a problem.
There are infinite shapes that cannot fall down the matching hole, but the circle is the easiest to manufacture. A circular cover is easier to manufacture than e.g. a Reuleaux triangle shaped one.
It's not arbitrary. "Because it can't fall in the hole" is a compelling geometric and safety reason. The circular manhole cover is the simplest shape that has this property.
If they weren't round, superheros couldn't use them as frisbees/weapons.
If a car hits them and they bounce and spin, they will still (most likely) settle back into the hole correctly. Even a Reuleaux polygon won't always do that.
There are no points more highly stressed than others. Put an extreme amount of weight on them and the force is evenly distributed around the circumference, rather than overly stressing the corners and bending the cover (on a square lid).
You can make a cover of any shape you want that won't fall in. It depends on how much the cover overlaps the hole. Even with a round cover, you can't have zero overlap or else the cover would fall in.
Now back to the correct answer, which is because the manholes are round - that begs the question of why manholes are round. It's probably a matter of equally distributing the sideways forces from the dirt plus a matter of being able to manufacture round tubes more easily than, say, square tubes or hexagonal tubes.
pffft, what a silly question. Manhole covers are round because manholes are round...why would you make a square cover for a round hole?
(Seriously though there's a lot of conceivable reasons, and while that's one I doubt it's a major one, a lip prevents it. I think the fact that a circle doesn't need to be "lined up" and that they are easier to move, etc are much more important)
It's designed with a particular use-case in mind. When people complain that square pegs don't fit into round holes, it makes more sense for them to step back and evaluate what they are trying to accomplish, and the tools they are using the accomplish it.
First off, there will be a small lip to keep it from falling in flat. This means that the hole is actually smaller than the manhole cover by an inch or more.
Second, no matter how you orient an equilateral triangle with on point facing down the hole, there is an edge across the top that is too big to fall in. The way to visualize this, if you take one edge of an equilateral triangle and sweep it in a circle around one end, it will be outside of the triangle at all points except where the corners meet. So trying to drop it in any position other than with the edges lined up will make it impossible to fall through. And since there is a lip when the edges line up, it can't fall through.
In contrast with a square or rectangle, if you sweep one of the edges it will be inside of the square or rectangle at some points (on a rectangle, you have to use the small sides). This means all you have to do is shove one edge of the cover down the hole and it will fall in.
> The real answer is because manhole covers are mass produced and manufacturing circular ones is usually easier and cheaper than the alternatives.
My understanding is the primary reason is because circular covers cannot fall into the hole itself, whereas that problem exists with other shapes like squares.
My theory: They're heavy, so making it a circle at least eliminates the need to "align" it when putting it back. You can basically just drag it using a hook and it'll slot itself into place.
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