Manhole

Once, there was a work going on inside a manhole. A young worker by name Mahaadev was cleaning the surface of the manhole lid. A girl of almost the same age as Mahaadev going nearby giggled at him. Angered by this, Mahaadev asked her a simple question that generally why is manhole lid circular in shape? The girl could not answer this. So Mahaadev convinced her that everyone will be having self respect. He also told her to respect all people irrespective of their work. Finally the girl said sorry to him and promised him that she would never do it again.

Do you know the answer for Mahaadev’s question?


Hint 1

Note down in a sheet of paper whatever comes to your mind. List some 5-7 reasons (logical) and analyze on that.
When you get/go through the solution, you can know how your way of thinking differs or is similar to that of others.


Hint 2

The problem cannot be solved in one shot. Do a bit of literature survey and then think again.


Answer

Manhole covers are round because the manhole tubes/rings (or holes themselves) are round.


Solution

The problem is one of the most favorite questions for an interviewer. An interviewer generally will not be interested in the exact answer. The interviewer would like to know about the interviewee's thinking process, his problem solving capability, his approach for a common but untackled question, the application of his diverse skills etc. Here we shall discuss and analyze the possible answers and the best suiting answer.

The problem asks us to reason for the circular/round shape of the manhole cover/lid. There are many similar questions to the given question. Some of them are: Generally,
-> Why are the tiles rectangular/square in shape?
-> Why are the large boards (advertisement boards) rectangular/square in shape?
-> Why are the buildings built on a rectangular/square site?
-> Why are the stadiums circular in shape?
-> Why are the parks oval/elliptical in shape?
These questions are easy to answer. It is now high time to solve the given question which is a bit difficult one. Let us start from the fundas (fundamentals).

A manhole is a hole or opening through which a person can gain access to underground utilities such as sewerage, drainage, gas, telephone, electricity etc for maintenance.
A manhole cover is a lid to a manhole to prevent anyone falling in and to keep unauthorized persons out.



The many answers which compete for being the most appropriate answer for the problem are:
a) Covers are round if manholes themselves are round i.e. the cross-section of manholes are circular.
b) The circular cover cannot fall through the circular opening, whereas covers of other shapes can fall.
c) Circular covers do not need to be rotated to align them when covering whereas other shapes require orientation.
d) Circular covers can be more easily moved by being rolled.
e) Circular covers are easy to manufacture.
and many more...

Let us go into depth...

a) Covers are round if manholes themselves are round i.e. the cross-section of manholes are circular.
According to our analysis the given reasoning is the most logical answer. There are sites which give a number of reasons for the manhole cover being circular. Here we discuss why we think that the given answer may be the actual answer.

When we ask a person to dig a hole of say 3 meters depth, he will dig a hole as requied. Generally the hole will be of round shape. This is because it is very easy to dig holes of round shape. Also in many cases we just think of circular shapes. Now if that particular person is asked to fit a lid for that hole, what shaped lid will he use. Its obvious. He will use a circular shaped lid.

A cover is designed to fit a box/jar/container/tube/hole etc. Instead, the box is not designed to fit a cover. As is the shape of the box is the shape of the cover. So a cover assumes the shape of the circle if the hole/tube/box is circular.

Why are the tubes made circular is a very good question. There are many reasons for this.
-> Tendency and Tradition (Most Important).
-> Due to extreme environmental conditions, the tubes may contract or expand. If the tubes are round, contraction or expansion is uniform on all sides whereas for other shapes it is not so.
-> Round tubes are the strongest and most material-efficient shape against the compression of the earth around them.
-> Round holes are easy to dig.
-> Round tubes are easy to manufacture.

So we conclude that the aforementioned reasoning is the most logical and most fundamental solution.

Please Note: It is most common to use round manholes. There are non round manholes also. They are generally of shapes square or rectangle or in rare cases triangle.

b) The circular cover cannot fall through the circular opening, whereas covers made of other shapes can fall.
Yes. This is a mathematical fact. But can we think that this is the best answer for the problem. Many sites say so. These mathematical facts are called the advantages of the prevailing answer.

'Oh! the circular cover can never fall into a circular opening! So I will dig a round hole.' Can anybody think like this??? No. This is not the chronological order in which the process of creating a manhole takes place.

The thought process for creating a manhole is:
-> Think about the shape of the manhole and dig.
-> Think about how to cover the manhole in the best possible way.
The reverse order is not a good idea.

Please Note: Circle is not the only shape that cannot fall into its hole. There is a family of such shapes called Reuleaux polygons. The common feature among these shapes is that they have a constant width or diameter. It is more difficult to manufacture them when compared to a circle. Find out more about them.

So we conclude that the statement is one of the best advantages for the cover being circular but not the reason itself. Similarly the statements c) d) and f) are also very good advantages for the cover being circular.

From the above discussions we conclude that...
Manhole covers are round because the manhole tubes/rings (or holes themselves) are round. :)
We cannot say that Mahaadev knew whatever we discussed till now but its true that he had some knowledge about the manholes.