## Why Manhole Covers Are Round ?

Statistically, by now you have at least seen more than 100 manholes in your life time if your age is more that 25. Out of all these, did you see any manhole cover which is not round ? probably you have but not as many as round manhole covers. But why most of these manhole covers are around then ? There is mathematical side of these manhole covers being round.

**Why Manhole Covers Are Round ?
**

Of course being round makes them easy to roll and slide them into a place in any alignment. This is not the only reason though, the more compelling reason has to do with geometric properties on circle and other shapes.

Imagine a square trapped between two parallel lines.

As square rotates, lines push apart when square diagonal point touches these lines and comes back to original position when square edges touches these lines.

Now try same with the circle and lines stay exactly the same distance apart as they were originally at.

**Mathematically this property is called as a curve of constant width.
** Another such shape is
Reuleaux triangle
Similarly, any polygon with odd number of sides can also be used to generate constant width curve based shape.

Coming back to original question, short edge of rectangular manhole cover could line up with the larger area and fall right in but shape with constant width won’t fall in any orientation. Usually these shapes are circular but in theory, any constant width shape should work. So now you know why manhole covers are round.

UPDATE : One of our reader just pointed out that using rectangle manhole cover is possible and falling by the smaller edge problem can be avoided with following types of manhole covers.

Watch following video to know more on constant width shapes

[youtube=https://www.youtube.com/watch?v=wDBb2_I-oC4&w=602&h=350&rel=0]

Courtesy :
TED-Ed