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# Round vs. Square

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Round vs. Square - Back to the River Crossing Puzzles

Why is it better for manhole covers to be round rather than square?

This old topic is locked since it was answered many times. You can check solution in the Spoiler below.

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Round vs. Square - solution

You can turn a square manhole cover sideways and drop it down the diagonal of the manhole. You cannot drop a round manhole cover down the manhole. Therefore, round manhole covers are safer and more practical than square ones.

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another interview question =)

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The obvious answer is that manhole covers should be round because manholes are round.

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Correct-a-mundo!!! There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole. None what-so-ever! It's funny to think that even Manhole Covers were thought out well before mass production began. You would just think they were round because they were going on a ROUND HOLE...lol

Also, seeing as how the damn things weigh about 38 gazillion pounds, trying to lift one is not an easy task. The method to move them typically is "Rolling them". Needless to say, trying to ROLL a 4 edged square hunk of metal would

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Correct-a-mundo!!! There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole. None what-so-ever! It's funny to think that even Manhole Covers were thought out well before mass production began. You would just think they were round because they were going on a ROUND HOLE...lol

Also, seeing as how the damn things weigh about 38 gazillion pounds, trying to lift one is not an easy task. The method to move them typically is "Rolling them". Needless to say, trying to ROLL a 4 edged square hunk of metal would <!-- s:oops: --><!-- s:oops: -->

Veracity, the veracity of your statement is suspect:

"There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole." , You couldn't be more wrong!

heard of "constant diameter curves" ?

A circle is just one of many such curves:

and This

That said, you are bang on target with your second statement about the ease of rolling

R

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Correct-a-mundo!!! There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole. None what-so-ever! It's funny to think that even Manhole Covers were thought out well before mass production began. You would just think they were round because they were going on a ROUND HOLE...lol

Also, seeing as how the damn things weigh about 38 gazillion pounds, trying to lift one is not an easy task. The method to move them typically is "Rolling them". Needless to say, trying to ROLL a 4 edged square hunk of metal would

Veracity, the veracity of your statement is suspect:

"There is not a single shape (besides perfectly round) that could not be positioned in a way that would cause it to fall in the hole." , You couldn't be more wrong!

heard of "constant diameter curves" ?

A circle is just one of many such curves:

and This

That said, you are bang on target with your second statement about the ease of rolling

R

Well, Look at that.. Your right. I stand corrected. I love learning new things. Thank you for taking the time to add a link to back your reasoning up....!!

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While it may not be correct that it is the only shape that won't fall through the hole, it appears to be the only one that won't fall through and actually serves the purpose of a manhole cover -- to cover the manhole. Of course, if you were just trying to prove someone wrong so you looked smart, congrats!

Dave

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While it may not be correct that it is the only shape that won't fall through the hole, it appears to be the only one that won't fall through and actually serves the purpose of a manhole cover -- to cover the manhole. Of course, if you were just trying to prove someone wrong so you looked smart, congrats!

Dave

LOL

I wasn't going to say anything figuring if I had it would look like I was Making Excuses to Justify my being Incorrect... But since you mentioned it...lol

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One of the English' coins is a condiam (constant diameter curve).

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a round lid covers more area of the square then a square lid to help from it leaking (why they dont want it to leak its a sur) and you can roll it out of the way making it easier.

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They condiam coins are the 20p and 50p ones.

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a triangle shaped cover wouldnt fall into a triangle shapped hole either

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a triangle shaped cover wouldnt fall into a triangle shapped hole either

i was about to say this however i should add that it's only an equilateral triangle that won't fall through.

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a triangle shaped cover wouldnt fall into a triangle shapped hole either

i was about to say this however i should add that it's only an equilateral triangle that won't fall through.

While you could create a cover/hole combination where a triangle wouldn't fall through, it is not a certainty.

It all depends on the lip width and thickness of the cover material.

An equilateral triangle has a maximum diameter (the length of a side) that is ~1.15 times the minimum diameter (center of a side to the opposite vertex).

```  ______

|		  /

|		/

x	  /

|_   /___________

|--1.15x--|

- Dimensions```
So as long as the lip isn't too wide and the cover isn't too thick, it will fall through. Turn the cover so a one side of the triangle is vertical and the bottom or top is flat against a side of the hole.
```		  /|

/   |

/	  |

|

|

|

~~~~~~~~~~~~

(long side of hole)

- Side view

/		<- hole

/

/

/ ==========	<- cover (===)

~~~~~~~~~~~~~~~~

-Top view```

In any non-equilateral triangle the ratio of maximum diameter to minimum diameter gets larger and it is easer to fit the cover through the hole.

(sorry for the ugly drawings)

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Well, I'd imagine because you can't fit a square into a circle.

And if they didn't have a manhole itself as a circle, and had it as a square, maybe it wouldn't be right because people who climb down there are generally fat? No idea on that one there.

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ahhh this is the stupidest question ever!

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Its round either because people are round, and its easier to traverse, or that pointy manhole covers would bend/break.

-=society works cause we think it does=-

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There are many answers I believe some of them are of which is they are safer with no points/verticies to poke people. Fits in any orientation, square won't if you rotate them 45 degrees. As well as the others already mentioned

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i have the perfect answer. they arent square so you dont stub your feet on them if they stick up too much.

lol!!

oh and haha i though this was a messed up face not a lightbulb haha

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square would be better in my oppion because you can put hinges on it easily.

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But hinges would either create an uneven driving surface if it was designed to have the cover flip 180 degrees, or the cover would be stick strait up perpendicular to the road. This would be a hazard since it could slam on someones head, or could make a driving hazard.

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Why would they want hinges on it anyway? it would be way to hard to lift.

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But what about a semi-circle? A shape that will not fall through the shape of the hole...

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Bingo excellent logic

Moreover in a unit dimension

a round has more are than a square

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A circle or condiam (continuous diameter curve) aren't the only shapes that aren't susceptible to falling through the hole: any triangle would be safe from this too. But climbing through a triangular hole would just be silly; and moving a triangular cover would be inefficient as well.

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