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        riddles >> microsoft >> WHY R MANHOLES ROUND
        
(Message started by: a_noctor on Nov 21st, 2002, 4:12am)
Title: WHY R MANHOLES ROUND
Post by a_noctor on Nov 21st, 2002, 4:12am
>:(
Title: Re: WHY R MANHOLES ROUND
Post by william wu on Nov 21st, 2002, 2:46pm
Hi Mr. Noctor; welcome to the forum. The manholes riddle was posted in the medium section of the website, so you can find discussion about that problem in the medium section of the forum. Here is the thread:

http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_medium;action=display;num=1027805679

To summarize, there are multiple good reasons for why manholes are round, but one of the best reasons is that that's the only shape which won't fall into the hole.
Title: Re: WHY R MANHOLES ROUND
Post by john on Dec 14th, 2002, 9:24am
An octagon?
Title: Re: WHY R MANHOLES ROUND
Post by Graham on Feb 18th, 2003, 7:27am
Yes this isn't strictly true - an equilateral triangle won't fall through either...
Title: Re: WHY R MANHOLES ROUND
Post by James Fingas on Feb 18th, 2003, 7:54am
Graham,

An equilateral triangle could fall through. Notice how the height of the triangle is smaller than its base, so turn the cover so the height fits into the base of the hole. Whoops!

There are other shapes that wouldn't fall, however: the shapes of constant width. The circle is the simplest of these shapes.
Title: Re: WHY R MANHOLES ROUND
Post by ShivaM on Feb 19th, 2003, 8:17am
If the cover is round then is easier to move it by rolling it on the ground.....
Title: Re: WHY R MANHOLES ROUND
Post by grimbal on May 1st, 2004, 9:18am
I would ask: why not?  Round is the simplest and most common shape for a hole.  Do you think a square hole would be more useful?  Wells are round also, even though they don't have a cover.

Regarding what shape does not fall in its own hole, a Y-shaped hole, with 3 branches of the same length and angles 120° would also be safe from falling if the branches are thin enough.  Of course, we should consider only convex shapes.
Title: Re: WHY R MANHOLES ROUND
Post by Dagny on Oct 7th, 2006, 11:34am
Round cover wud be safer too. It wont hav corners n thus tere r less chances of getting hurt while manipulating the cover.
Title: Re: WHY R MANHOLES ROUND
Post by irrational on Nov 1st, 2006, 8:19am
Another reason is:

for a given area, circle has the least perimeter and hence would take the least space.
Title: Re: WHY R MANHOLES ROUND
Post by Whiskey Tango Foxtrot on Nov 1st, 2006, 6:19pm
Or that it is the easiest to manufacture.
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Nov 2nd, 2006, 7:01am
The standard answer is:
- It is the only shape (so they say) that prevents the cover to fall in the hole.
- It is easier to move around, by rolling.
- It is easier to put back because it can be put in any direction.

Here is a picture of a manhole cover near my place note that it is in 2 parts and is on hinges.

http://florian.net/pic/misc/manhole.jpg

Let's see:
- Does the shape prevent the cover to fall in the hole?
- No, it is the hinges.
- Is it easy to move around?
- No.  It is round but it cannot be moved around.
- Does the shape make it easier to close?
- No, it's the hinges.
- Is it round?
- Yes
- WHY?
Title: Re: WHY R MANHOLES ROUND
Post by TenaliRaman on Nov 15th, 2006, 10:31am

on 11/02/06 at 07:01:10, Grimbal wrote:
- Is it round?
- Yes
- WHY?

It looks pretty, i wish i could have one too!

-- AI
P.S -> This answer wont get you the job.
Title: Re: WHY R MANHOLES ROUND
Post by BNC on Nov 21st, 2006, 4:15am

on 11/02/06 at 07:01:10, Grimbal wrote:
- Is it round?
- Yes
- WHY?


It may be worth repeating what our lost mate Kozo said in the other thread:

on 08/06/02 at 16:45:10, Kozo Morimoto wrote:
According to a civil engineer friend who actually designs the damn things, the dropping theory is pretty cool, but the main reason is probably to do with the same reason why portholes on ships are round - to distribute the load stress evenly and avoid stress points.  When you have heavy loads (ie trucks and buses on roads driving over manholes constantly) having 'pointy' shapes would reduce the life of the manholes and increase maintenance requirements.  If the dropping thing is correct, why don't they just attach the covers with cables/hinges or make them slide across rather than be able to completely remove them?



I also like the (made up) interview with Richard Feynman :

Quote:
Interviewer: Now comes the part of the interview where we ask a question to test your creative thinking ability. Don't think too hard about it, just apply everyday common sense, and describe your reasoning process. Here's the question: Why are manhole covers round?

Feynman: They're not. Some manhole covers are square. It's true that there are SOME round ones, but I've seen square ones, and rectangular ones.

Interviewer: But just considering the round ones, why are they round?

Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology.

Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones?

Feynman: Yes. Round covers are used when the hole they are covering up is also round. It's simplest to cover a round hole with a round cover.

Interviewer: Can you think of a property of round covers that gives them an advantage over square ones?

Feynman: We have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it. Also, the term "manhole" implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section. So a cylindrical pipe is the natural shape for manholes. The covers are simply the shape needed to cover up a cylinder.

Interviewer: Do you believe there is a safety issue? I mean, couldn't square covers fall into the hole and hurt someone?

Feynman: Not likely. Square covers are sometimes used on prefabricated vaults where the access passage is also square. The cover is larger than the passage, and sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. Let's assume a two-foot square opening and a ledge width of 1-1/2 inches. In order to get it to fall in, you would have to lift one side of the cover, then rotate it 30 degrees so that the cover would clear the ledge, and then tilt the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. Yes, it's possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely. Applying common engineering sense, the shape of a manhole cover is entirely determined by the shape of the opening it is intended to cover.

Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.)

(Interviewer returns after 10 minutes)

Interviewer: We are going to recommend you for immediate hiring into the marketing department.
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Nov 21st, 2006, 4:34am
Manhole round because mandick round.  :-X
Title: Re: WHY R MANHOLES ROUND
Post by balanagireddy.tcs on Nov 25th, 2006, 2:11am
I found a reason why manhole is round...

Basically manholes can be of any shape.

But we need to find something to minimize the area because iron is not that cheap.

and circle is the best possible shape.

Many more shapes like equilateral triangle etc...are also possible.

the reason for manhole is to avoid things fall into those holes.

thats why again circle is the best.

MAN HOLES SHAPE SHOULD BE THE SHAPE OF THE HEAD TOP VIEW.

In future if cubical faces exist...then there will be square manholes...
Title: Re: WHY R MANHOLES ROUND
Post by checker on Nov 25th, 2006, 2:30am
I am really surprised to know that such a simple question could be a problem.......When I was asked this in my school days that time I would have found Bala right...but for today Kozo is having the right approach....Well for me every common thing carries a lot of uncommoness.
Title: Re: WHY R MANHOLES ROUND
Post by srn347 on Sep 8th, 2007, 8:43pm
Because if they were square(or some other shape) it would be harder to explain why it is.
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Sep 9th, 2007, 10:26am
I have to make a picture, But I have seen triangular ones!
Title: Re: WHY R MANHOLES ROUND
Post by JiNbOtAk on Oct 5th, 2007, 6:56pm
I hate to break it to you guys, but most ( if not all ) of the manholes here in Malaysia is rectangular. In fact, round manholes kinda like endangered species - you know they exist, but its really hard to spot one.
Title: Re: WHY R MANHOLES ROUND
Post by upen_iitr on Oct 27th, 2007, 2:46am
since for given dimensions circle has maximum area and
pressure=force/area

hence area being maximum reduces the pressure n th8s possibly the reason behind manholes being round..
Title: Re: WHY R MANHOLES ROUND
Post by spazdor on Nov 5th, 2007, 8:59pm
Actually there's lots of shapes that won't fall in.

Probably the simplest such shape (other than the circle) is constructed as follows:

Construct equilateral triangle ABC.
Draw circular arc AB about center C.
Draw circular arc AC about center B.
Draw circular arc BC about center A.

The shape outlined by those three arcs also won't fall down a manhole.
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Nov 6th, 2007, 12:45am
It's called the Reuleaux triangle.
http://mathworld.wolfram.com/ReuleauxTriangle.html
Title: Re: WHY R MANHOLES ROUND
Post by JiNbOtAk on Dec 17th, 2007, 4:24am

on 10/05/07 at 18:56:45, JiNbOtAk wrote:
I hate to break it to you guys, but most ( if not all ) of the manholes here in Malaysia is rectangular. In fact, round manholes kinda like endangered species - you know they exist, but its really hard to spot one.


Sorry guys, I made an error. According to my wife, the round manholes are for electrical and phone lines. The rectangular ones are for sewage. I guess I'm so used to see the rectangular manhole covers, I kinda blank out on the round ones.
Title: Re: WHY R MANHOLES ROUND
Post by tiber13 on Dec 19th, 2007, 1:19pm
wouldn't a triangle be kinda hard to get through, unlike a circle
(at small sizes)
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Dec 20th, 2007, 6:50am
hard, but not impossible.
Title: Re: WHY R MANHOLES ROUND
Post by BNC on Dec 30th, 2007, 3:10am

on 09/09/07 at 10:26:00, Grimbal wrote:
I have to make a picture, But I have seen triangular ones!

Picture in link (http://www.drainspotting.com/mirror/nhtelegraph-2003-11-26/)
Title: Re: WHY R MANHOLES ROUND
Post by meegers on Jan 26th, 2008, 5:26am
Why are manhole covers round?

Well, why are manholes round? Because the fastest most efficient way to create a long deep hole on the ground is by drilling or boring, which naturally results in a round  tunnel. the easiest and most efficient way to cover a round tunnel? Round lid.
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Jan 26th, 2008, 2:25pm
I have never seen a hole dug in the road by using a giant drill.
Title: Re: WHY R MANHOLES ROUND
Post by MakingaSplash on Oct 10th, 2011, 3:50am
Erm, I hate to throw a spanner in the works but manholes in the UK are normally rectangular!!

Mike
Irrelevant link removed --SMQ
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Nov 27th, 2012, 6:03am
I'd like to add another dimension of thought to the whole question.

They are round because round manhole covers look cute :) .
Title: Re: WHY R MANHOLES ROUND
Post by Grimbal on Dec 13th, 2012, 12:56pm
And I can prove it.
http://florian.net/pic/misc/cutecover.jpg
Title: Re: WHY R MANHOLES ROUND
Post by SMQ on Jan 21st, 2013, 4:04am

on 12/13/12 at 12:56:52, Grimbal wrote:
And I can prove it.

Did you really update your avatar script to serve that image just on this thread? Nice. :)

--SMQ
Title: Re: WHY R MANHOLES ROUND
Post by towr on Jan 21st, 2013, 9:00am
I don't think he needed to update the script for that. I think he has one (usually randomly chosen) avatar per thread anyway, so it's just a matter of setting the right one.
Title: Re: WHY R MANHOLES ROUND
Post by SMQ on Jan 21st, 2013, 10:20am
Not to get too far off-topic, but I believe Grimbal's avatar script is normally just random, not thread-based. If I bring up another thread he's commented on (e.g. this one (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_easy;action=display;num=1358514414)) and hit refresh I get a different avatar each time.  On this thread, however...
Title: Re: WHY R MANHOLES ROUND
Post by towr on Jan 21st, 2013, 11:23am
Seems I misremembered a conversation (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=what_am_i;action=display;num=1227999941;start=5#5), but at least I remembered it took place :P


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