Why are manhole covers round? Yes I know...because the holes are round. But why round? Anyone?
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M$4 Answers
Secondly, manhole covers are HEAVY - I believe they're made of cast iron, or something similar. A round manhole cover can be rolled by one person if it needs to be moved - other shapes may require more than one person to carry.
They're easier to line up as they fit in any way you turn them
It's easier to dig a round role or create a round pipe than it is to dig a square hole.
A round shape requires less material (volume) to fill the same dimensions than any other shape - mathematically for the same reason that bubbles are spherical.
The typical "correct" answer is the first, but any others will also work well. This is always an interview question supposed to test "critical thinking" - most people know the FIRST answer either from preparing for interviews or from being told in an interview (which is how I learned). If you are trying to impress an interviewer, use some of the other answers - such as the fact the round shape is easier to move and requires less materials. This shows you are thinking, and it's less likely to be from straight memorization because it's not the common classic answer - but it's still right!
Good luck!
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M$
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M$
Huh.
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M$
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M$
How many men do YOU know that are triangular? Sheesh!
If I had been presented with that question at an interview, I wouldn't've been hired. I'm logging that factoid away for future reference...
They explained this on Mr. Wizard's World once!
Someone mentioned the Reuleaux triangle below. Believe it or not, for a given diameter, this shape has a smaller area than a circle. In fact, it's the smallest area of all with a given constant width.
http://en.wikipedia.org/wiki/Reuleaux_triangle#Other_uses
The triangle manhole cover can be dropped into the manhole.
Flip the manhole cover 90 degree along the line of a tip and middle of its opposite edge, drop it at one of the 3 edges of the trianglular manhole.
Oddly enough in the city that I live in we have the rare but interesting Triangular Manhole Covers..said to serve the same purpose as the Circular Manhole Covers but that is open to interpretation.
- http://www.waymarking.com/waymarks/WM4FTF_Triangular_Manhole_Cover__Nashua_NH
- http://www.straightdope.com/columns/read/220/why-are-manhole-covers-round
I just gave it a try (because i was curious) by cutting out a triangle from a piece of box board. Unless I'm really bad with scissors, it CAN go through.
I didn't trust my cutting abilities, so I did the math. In paint. So I can share.
If the length of one side is 100 units, from tip to middle of the opposite side would be 86 units, meaning if you flipped it on it's end and rotated it 90 degrees it would slip right through the hole. Unless it had a pretty big lip on it - it'd have to be bigger than 15% of the width of the base of the triangle. That's quite a bit - for a 12 inch manhole cover, thats an inch on either side. For a 12 inch manhole cover, that's 2 inches on either side. 2 inches on either side would be unreasonable.
Sir, I challenge your claim that the triangle cannot fit through it's own hole!
I'd have to do the math on that one! I'd think you could by putting the tip in and rotating it through but I'm not sure that is true. Mathematically it might work but I'd hate to be the one to dig a triangular hole!
@potterarchy - Now you know! To be fair, I didn't know it on an interview either and I still got the job... I'm really not sure they actually base anything on your answer, but an awesome answer could still likely impress them!
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