Why are diesel driven vehicles noisy?

Diesel engines are inherently noisy because of the auto-ignition of the initially formed mixture of fuel-vapour and air, which causes rapid rate of pressure rise producing the characteristic noise. This abnormality is due to the inborn feature of a diesel engine, which uses a high compression ratio to obtain high fuel efficiency and a high compression-temperature for ignition of the fuel injected into the cylinder at high pressure. In a diesel engine, unlike a petrol engine, the air and fuel do not mix outside the engine.

Air is compressed to a high compression ratio leading to high fuel efficiency, and also to a high compression temperature to initiate combustion. They just meet inside the combustion chamber for a brief period, while all the processes of mixture formation like fuel-jet break-up, evaporation and mixing should take place within a short time called ignition delay period. Combustion follows this after the initially formed fuel-air vapour auto-ignites with a noise, forming the sources of ignition (chemical spark plugs if you wish to call) for the bulk of the fuel remaining. Diesel engine can be made less noisy by using common rail high-pressure injection system and electronic control. Courtesy : The Hindu

Why does a mushroom shaped cloud form after a nuclear bomb explosion?

ANSWER I: When a nuclear weapon explodes, there is a rapid release of a large amount of energy within a small volume. This results in significant increase in temperature and pressure. The temperature may be a few tens of million degrees and pressure a few million times the atmospheric pressure. At this temperature, all the material present in the weapon will be converted into hot compressed gases.

Within a fraction of a millionth of a second of the explosion, the weapon's residues emit large amounts of energy mainly in the form of X-rays. The surrounding atmosphere absorbs this energy. This results in the formation of a blazing, highly luminous, spherical mass of air and gaseous weapon residues called the fireball.

Within an extremely short time after the explosion, the fire ball from a high yield nuclear weapon will be about 130 metres across increasing to about 1700 metres in ten seconds.

The fireball expands rapidly engulfing the surrounding air. The ball of hot air is less dense than the surrounding air. It rises swiftly like a hot air balloon.

This rising column pulls up debris of the weapon, dust and moisture along with it forming a cloud. As it moves up, it cools gradually and reaches about 10 km where the atmosphere is extremely stable.

The ball of air mass moving up does not have enough energy to penetrate this stable layer. It flattens out. As the relatively warmer layers at the bottom push up, the top layers spread laterally and equally in all directions, and the cooler denser layers descend at the edges, giving a distinct mushroom shape.

ANSWER II: Atmospheric nuclear explosion leads to sudden formation of a massive fireball near the ground, setting aflame whatever is in its vicinity. Since the fireball is very hot and thus less dense than the surrounding air, it rises very quickly. The massive updraft due to the rapidly rising fireball leaves a column of low-pressure. This acts as a chimney, sucking in smoke, dust and debris from the surroundings. This forms the stem of the mushroom.

At first the mixture of hot air and dust rises vertically, forming the column of the cloud. But as the hot cloud meets the colder air at higher altitudes, it slowly cools. Eventually the cloud reaches the temperature of the surrounding air and ceases to rise, but spreads horizontally along air levels at the same altitude, which are at the same temperature. This forms the cap of the mushroom.

The smoke, dust and debris gushing into the central column cause toroidal eddy currents in the horizontally spreading hot cloud. This introduces curling under the cap of the mushroom. Mushroom clouds are most commonly associated with nuclear weapons. However, any massive explosion capable of creating the same conditions would produce a mushroom cloud. Volcanic eruptions are typical natural mushroom clouds. Courtesy : The Hindu

Why are we unable to walk straight and tend to lose our balance when we walk with our eyes closed?

ANSWER I: When one wants to walk in a straight line the brain takes a visual reference point and uses this reference point to maintain the movement in a straight line. In the absence of a visual reference point the brain is unable to ascertain the exact direction in which the body is moving and hence most of us will be unable to walk in a straight line with our eyes closed.

However visual reference is not the only mechanism of maintaining balance or direction in our movements. We can judge the direction of movement very accurately with the balance sense from the balance mechanism in our inner ear and by kinesthetic sense, which is sensations we derive from our muscles, joints and ligaments.

Both these sensations will tell us very accurately as to the balance and direction of movement and position of the body at any point of time. This is highly developed in acrobats, gymnasts and ice skaters. They would be able walk in a straight line even with their eyes closed.

ANSWER II: Human balance is maintained by three pillars namely vestibular system — balance organ in the inner ear and its connections — eye and proprioceptors in the joints of the body. Man can maintain balance with any two pillars but not with one pillar alone. Imagine a blind man who can walk without losing balance.

If the same person develops any disorder in the vestibular system or proprioceptive system, then he cannot stand or walk. Similarly if a normal person develops a disorder in the inner ear or proprioceptive system and tries to stand with eyes closed and feet together, he will lose balance. Courtesy : The Hindu

Why do pregnant women like eating tamarind and raw mangoes?

Craving for unusual food (and some non food items like ash called pica) is considered as the first sign of pregnancy. In reality, although some women do get strong cravings, many do notNo one knows for sure what causes food cravings. Many women find that their senses of taste and smell are changed by pregnancy. For example, some women experience an odd metallic taste in their mouths very early in pregnancy (maybe the first sign of pregnancy for an `experienced' mother); others find that taste and smell are dulled. It is possible that these changes affect food likes and dislikes.

Some people think that cravings happen in response to temporary deficiency of specific nutrients. There is probably some truth in this, but it is not the whole story. We only need minute quantities of each vitamin and mineral — certainly not enough to justify a continual craving for just one food. For some women, food cravings may be a conscious or subconscious response to emotion.

They may crave a favourite childhood food, or a food that is of special significance to their religion or culture. Craving unusual foods may also be a private way of marking the special state of being pregnant.

Rather than develop a food craving, many women find they suddenly go off certain foods or drinks like fried foods and coffee.

This is often related to pregnancy sickness, but may also be the body's way of ensuring that they eat and drink wisely. Generally, there's no harm in giving into food cravings, especially if doing so helps getting through phases like early morning sickness which can be pretty distressing. However this must be done in moderation.

Eating a lot of one food only means eating less of other foods and therefore running the risk of becoming deficient in important nutrient. A craving for non-food items — such as ash or soap or toothpaste — is known as `pica'.Pica is potentially very harmful if indulged in and must be resisted.

Also, substances like soaps and ash may prevent the absorption of nutrients and other food substances. Courtesy : The Hindu

Why do doctors prescribe some medicines to be taken before and some after food?

When we take a medication, it is absorbed from various parts of our gut — some get absorbed in the stomach, some pass through the stomach into the intestines and get absorbed there.

The most important reason for timing a medication that is taken orally is to maximise its absorption so that more of the medication goes through the stomach into the blood.

Many medicines get absorbed better when food is not present and hence are taken on an empty stomach (an example is the hormone thyroxine which must be taken first thing in the morning). A few actually get absorbed better when food is present — an example of this is the antibiotic azithromycin. Some drugs are taken specifically with or after food because this may reduce the side effects of the drug on the stomach.

For example pain medications and certain antibiotics all can irritate the lining of the stomach and therefore are best taken with or after food. Some drugs work in the wall of the stomach to reduce the absorption of food and this is the desired therapeutic effect — an example of this is the anti-diabetic drug called acarbose.

This drug must be taken with the first bite of food. Similarly other oral anti-diabetic drugs and the injection insulin are taken before food because that is when they need to act — just after you eat a meal. The long and short of it is that the timing of a drug has important effects on its absorption, action, potency and even side effects and it is a good idea not to leave the doctor's office without being sure when you should be taking your medicine. Courtesy : The Hindu

A hen lays an egg every day. From where does it get the calcium required to make the eggshell?

 A commercial layer that lays an egg almost everyday derives its calcium requirements entirely from the feed it consumes. The nutritional requirements of chicken have been understood completely over the years. Today's layer bird yields 325 eggs in 365 days.

The chicken feed is mainly composed of maize, broken rice, soya, groundnut and sunflower cake, oyster shell grit and dicalcium phosphate.

The calcium requirements are mainly contributed by the shell grit. The feed is a precisely formulated one and the nutritional requirements of the layer are fully met by the feed it consumes daily. Courtesy : The Hindu

Why does it take a longer time to copy a file to a computer than delete it?

An Operating System (OS) is a program that acts as an intermediary between a user of a computer and the computer hardware.

Depending on the storage device being used, computers can store information in several physical forms. Each device has its own characteristics and physical organization, and hence different views of information are created. To unify all these views of information, a uniform logical view called a file is created.

A file is a contiguous set of data. It is the job of the OS to map this sequence of data into physical devices. The part of the OS responsible for this is the file system.

So the main task of the file system is to free the users of the details of storing of information in the physical devices. That is, when the storage device is changed, from disk to CD for example, the user still sees the same information.

In the most basic form, a file system consists of two distinct parts: a collection of files and a directory structure. The directory structure organizes and provides information about all the files in the system.

Every file has certain attributes like its name, location (its address in the file system), size, access control information (whether the file can be read or written to or only executed etc), type (whether it is just a collection of data or has other special instructions), time, date and user identification.

If file A is to be copied to file B, then first a new file called B is created, next the contents of A are read and finally, this is written to B.

For this to happen, three steps are necessary. First, space must be found for B in the file system. Second, an entry for B must be made in the directory. The directory records the name of B and the location of B in the file system. Third, a request is sent to the OS to read the contents of A.

The OS finds the location information of A from the directory and reads the contents. Now to write to B, the OS again searches the directory for the address of B. Finally the content of A is written onto the space provided for B.

On the other hand if a file deletion is to take place, the process is much simpler. If file A is to be deleted, the OS just searches the directory for the named file.

Having found the entry, the space occupied by A is released so that it can be used by other files and the directory entry is erased.

As can be seen, the operations involved while copying a file are much more than those involved while deleting a file. Courtesy : The Hindu