Can vehicle tyres be filled with liquid instead of air?

Vehicles are filled with air to absorb the shocks generated as the vehicle moves on uneven roads, and make the ride comfortable.The shock absorbing capacity is present by the fundamental property of air i.e compressibility - air can be compressed to smaller volumes on application of pressure.

But liquid is not compressible and so if one uses it to fill the tyres, all the shocks generated on the tyres will be completely passed on the vehicle and its passengers will feel the discomfort.In addition liquid filled tyres increase the rolling resistance  of vehicles because of the increase in weight. Hence a part of the engine's power and fuel is wasted in overcoming this resistance.

Why is fire hot?

Chemical reactions can be either exothermic(heat liberating) or endothermic(heat absorbing).Oxidation reactions are exothermic and reduction reactions are endothermic. Combustion is an oxidation reaction and hence is exothermic.
                                                 
All liquid, solid, and gaseous fuels contain any one of the three combustibles constituents viz. carbon, hydrogen, and sulphur. When fuel is burnt the heat liberated makes the products of combustion hot. The products of combustion are carbon dioxide, water vapour and sulphur dioxide. Along with these, the unused oxygen in the atmospheric air supplied for combustion and also the entire quantity of nitrogen which is the major constituent in the air are also heated. That is why fire is hot.

How does ballast-less track provide safe travel?

Ballast is a prime component of the track that acts as a vibrant medium to transmit from the rail surface the weight of the train down to the formation, the wells set on prepared terrain. It ensures a cushioned and smooth run for the train and precludes the longitudinal displacement of the rail called creep. In ballast-less track, as the name suggests the ballast is replaced by a bed of concrete.
          The rails rest on rubber pads placed over concrete sleepers, which are fixed on the concrete bed. The ballast-less track helps to eliminate the evils of dust and noise pollution and proves suitable for underground railway also. So it is also safe for travel.

Why are aeroplanes not affected by lightning?

Clouds are made of water droplets and dust in the atmosphere at altitudes of 1 to 16 km.While traveling through air,due to friction,the water droplets get electrically charged.Lightning occurs due to sparking between oppositely charged clouds-a high voltage spark rushes towards the ground(at zero potential) through moisture-filled air.

If any ground based structure is on the path of the spark,the top of that structure is excited to a high electrical potential while its bottom (in contact with the ground) remains at zero potential.This high potentail difference sets a very high current in the structure causing sudden heat generation in the material and destruction.But in flying aeroplanes,there is nothing like a ground point which remains always at zero voltage.

Hence even if hit by a lightning,the plane's entire surface acquires the same high potential and due to lack of potential difference there is no current and hence no destruction.

What constitutes bulllet-proof glass ?

High-strength ceramics such as silicon nitride,dual hardness steel and layers of heavy nylon fabric make a bullet-proof glass impervious to bullets.The hard ceramic stops the bullet abruptly,dissipates its energy as it destroys the first layers of the glass at the point of impact and entangles it in a coarse mesh.

A textile bullet-proof vest is fashioned of 16-24 layers of nylon cloth of heavy weave,the layers stitched together like a quiet.Such vests or full-torso protectors can be worn undetected under regular clothing.A vest of 16 layers will stop regular handgun and sub machine-gun bullets,those of 24 layers will stop the powerful magnum bullets from the same weapons.

Why should we not sleep north-south?

Researchers at the Chennai Institute of Magnetobiology, say that sleeping in the north-south direction could make one lack a sense of well being times. According to Dr. Sankarnnarayanan, director of the institute, when a man lies with his head towards north in a pulsating magnetic field, his brain's electrical activity is suppressed or damped. Measurement reveal that blood circulation in finger tips are affected and the balance between neurochemicals is upset. In addition, it gives subjective experiences like headache, confused thinking and lack of a sense of well being.

As oppose to this, he says, if one sleeps east-west, in the pulsed field, the brain's activity is considerably enhanced. Also, the peripheral blood circulation all over the body is enhanced. Hence he feels relaxed and alert.

It is known that the earth's magnetic field pulses at extremely low frequencies and is typically in range of human brain's electrical activity. These pulses are not present always but they peak during certain time periods generally associated with the solar activity. If one sleeps north-south during such activity headache, nausea and confusion may result. On inactive nights, there may not be any change in the body, he says. This is true for animals also as they are more sensitive to such changes.

Why do thin glass tumbler break when filled with very hot water?

Glass tumbler break due to the difference in thermal expansion of their inner and outer surfaces.Glass is known as a bad conductor of heat.Hence when the tumbler is suddenly filled with hot water,the inner layer of the glass gets heated immediately as it comes into contact with hot liquid.But the outer layer remains at a lower temperature for some more time as the heat is conducted through the glass only slowly.

As a result,the inner layer tends to expand more than the outer layer.This leads to cracking.Thick-walled tumblers and tumblers made of special quality glass withstand such cracking to a certain extent.