Answers to Last Issue's Do You Know?

1. At many modern libraries, the books are specially tagged with a radio device. If you try to leave the library without checking the book out, the alarm goes off. On the other hand, if you do take such a book to the check-out counter, you no longer have to fill in a card: the machine automatically registers the fact that the book has been withdrawn (and also identifies you as the person who withdrew it). How does such technology work?
Answer: This is because the book has an RFID tag attached to it. RFID (Radio Frequency IDentification) is a combination of radio-frequency based technology and microchip technology. The tag that is affixed to the book has a microchip inside it. The information on the microchip is read using radio frequency technology.
One advantage of using this is that the tag can be read without bringing the book into contact with any reader, and whichever way you tilt the book does not matter. Also, several books can be checked out very quickly without processing them one after another sequentially. This reduces work for the library staff.
One bit on the chip is designatd as the "theft" bit. If the book is checked out properly, this bit is turned off at that time. When the book is returned, the bit is turned on again before shelving.
So when the book is on shelf inside the library, its circulation status (namely, that it is not in circulation) is encoded on the RFID tag.
If the book has not been checked out, the bit is still on, so when it is taken past the exit, the sensors detect the bit and an immediate alarm is triggered.
This is the simplest system; some systems are sophisticated and carry more information in the RFID tag: alongwith triggering an alarm, they also identify which particular book has been taken.
RFID is useful not only for checking books out but also for normal library maintenance and inventory. Very often, books tend to be mis-shelved, that is, placed on the wrong shelf in the wrong rack. This naturally makes searching for a book very hard.
A hand-held inventory reader can be moved rapidly across a shelf of books to read all of the unique identification information. Using wireless technology, it is possible to identify items which are out of proper order.
Notice that in this way we can scan books on the shelves without tipping them out or removing them. This is very helpful for making an inventory: checking that all the books are present where they should be.
2. India became independent in 1947. Mahesh's mother was born in Goa in 1951 but was a Portuguese citizen. How can this be?
Answer: This is easily possible, since Goa was under Portuguese rule in 1951. Anyone born in Goa at that time, like Mahesh's mother, automatically became a Portuguese citizen. (Similarly, anyone born in Chennai in 1946 was a British citizen as well.)
The part of India which was being ruled by the British became independent on August 15, 1947. At that time, there were small parts of India which were not under British rule, and Goa was one of them. Jawaharlal Nehru asked Portugal to turn Goa over to India, but Portugal refused to do so.
In 1954, unarmed Indians took over the tiny regions of Dadra and Nagar-Haveli. This incident led the Portuguese to lodge a complaint against India in the International Court of Justice at The Hague. The final judgement on this case, given in 1960, held that the Portuguese had a right to these regions, but that India equally had a right to deny Portugal access to Goan territory.
From 1955 onwards, there were many incidents of demonstration against Portuguese presence in Goa, which were severely suppressed by Portugal.
On December 19, 1961, Indian troops crossed the border into Goa. Land, sea, and air strikes lasted for more than 36 hours; it resulted in the unconditional surrender of Portuguese forces. Under Indian rule, Goan voters went to the polls in a referendum. The people elected to become an autonomous, federally administered territory. Goa became a state in India only in 1987.
3. Why doesn't paper money disintegrate when it gets washed in the washing machine?
Answer: The paper used for currency is not like ordinary paper. Normally, the paper we use has been made out of cellulose, which comes from trees. This is true of notebook paper and newspaper. The biomass is chemically broken down into individual wood fibres, and the cellulose fibres are chosen and formed into very thin sheets to create paper.
Paper money, on the other hand, is made from paper made of rags. Cotton or linen fabric is beaten to create cotton or linen fibres. You have probably heard of "rag paper" or "fine linen writing paper." This is where the term comes from.
It turns out that rag fibres bond together much more firmly than fibres in regular paper. Rag fibres are basically unaffected by water, whereas cellulose fibres absorb water and come apart when they get wet. 
So paper money comes through the washer just fine, while cellulose paper comes apart.
4. In places with high-rise buildings such as in Mumbai, old buildings have to be demolished very carefully so that they do not fall on the adjacent buildings. How is this done?
Answer: You have probably seen a man holding a large-ish hammer and attacking a building wall to bring it down. When it comes to big buildings this is hardly sufficient. Worse, if this building is surrounded by other buildings, it is necessary to "implode" the building, that is, make it collapse down into its own floor space. This way it will not fall on adjacent buildings.
Strictly speaking, an implosion is any event where something collapses inward, because the external atmospheric pressure is greater than the internal pressure. For example, if you pumped the air out of a glass tube, it might implode. A building implosion isn't truly an implosion--atmospheric pressure doesn't pull or push the structure inward. It is gravity that makes it collapse. But the term implosion is in common use for this sort of demolition.
The basic idea of explosive demolition is quite simple: if you remove the support structure of a building at a certain point, the section of the building above that point will fall down because it has no support.
If this upper section is heavy enough, it will collide with the lower part with sufficient force to cause the process to continue, so that the entire building collapses.
Explosives are used to trigger or start the demolition. It's gravity that brings the building down.
Demolition blasters load explosives at several different levels of the building so that the building structure falls down on itself at multiple points. When everything is planned and executed correctly, the total damage of the explosives and falling building material is sufficient to collapse the structure entirely, so cleanup crews are left with only a pile of rubble.
The violent blasts and billowing dust clouds surrounding these spectacular implosions may look chaotic, but a building implosion is actually one of the most precisely planned, delicately balanced engineering feats one can get to see. In just a few seconds, the building neatly falls into itself, without damaging the adjacent ones.