If only this were true for certain utility holes in my neighborhood with rectangular lids. I I've sprained my ankle a few times. I mean, they won't fall in accidentally because of the lip. But you can lift one out and put it in the hole, due to negligence or a taste for vandalism.
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.
2023 answer: Excuse me? You mean a street-level maintenance access lid? So it can't fall in. And why are a bunch of the other similar lids not circle? I leave that one up to you vape hit
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.
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).
There is in fact a class of shapes that have the property that they can't fall into the hole which they cover (presuming a lip).
In addition to the above reason about it being the most efficient way to cover a hole, a round cover is also easy to move if heavy (you can roll it), very useful for something that likely has to be sturdy but also move-able.
Only with a sufficiently large lip. The diagonal of the hole has to be less than the shortest side of the lid. As I picture it in my mind this is an excessively large lip.
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.)
That is the argument but actually any lipped shape can give this guarantee. Even historically manhole and utility covers had to be somewhat flush to the ground and so they've always had to be lipped for this.
You can see lots of utility covers that are hinged, square, or other options, but all are lipped for safety. While a circular profile makes the lipping easier, it doesn't seem to influence many utility covers.
I'm fairly certain a square manhole cover with a sufficient perimeter would still have no chance of falling into a hole, depending on the diameter of said hole.
I don't know anything but how about putting it in a hole ?
What would be the consequences of that ? (Sure there are plenty but I am too dumb to think of any)
My guess would be that (many, most) manhole covers are round because it is the obvious shape for most lids. There are so many poorly designed urban objects that I tend to believe that the feature of not falling into the hole was a lucky accident.
Think of stop signs, for example. It would be really easy to have a different shape/color for four-way stops, but at least in the US all they do is add "four way" to the sign in an inconsistent way.
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