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MAY 03, 2014

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Q 1. Why do woollens keep us warmer during winter than cotton clothes?

A. The woollens have air in their pores. As air is a bad conductor of heat, it does not let the external cold air enter it nor does it allow the internal heat to escape.

Q 2. In case of a thunderbolt, lightning is seen first while sound is heard later. Why so?

A. The speed of light (3, 00, 000 km / sec) is far more than that of sound (332 m/s), enabling it to reach us earlier.

Q 3. Why does smoke come out when wood or coal is lighted up?

A. Wood and coal contain carbon-rich matter, which rises as smoke due to hot air during the lighting up of wood or coal.

Q 4. A hot liquid remains hot while a cold liquid remains cold in a thermos flask. Why?

A. A thermos flask consists of a double-walled glass vessel having vacuum between the walls. Heat cannot enter or leave the inner flask by conduction or convection through the vacuum. The heat loss by radiation is also prevented by silver-coating the inner side of the outer vessel and outer side of the inner vessel.

Q 5. Why do frogs bury themselves in damp places and become sluggish during winters?

A. As they cannot resist the intense cold due to their cold-blooded nature, they move to the damp place where the conditions suit them. This is known as hibernation or winter sleep. Such a phenomenon occurs during intense summers also and is called aestivation.

Q. 6. Why do animals need to eat food?

A. For normal, regular functioning of the body parts, the animals need energy, which is obtained from food.

Q. 7. Why does a blotting-paper absorb ink?

A. A blotting-paper has many fine pores, in which the ink rises up because of capillary action and gets absorbed.

Q. 8. Why do we feel more comfortable in white clothes during summers than in dark-coloured ones?

A. White-colored objects and polished things reflect heat well and are bad absorbers of heat while dark clothes absorb heat well. Thus, in winters, black and dark clothes are preferred as they absorb more heat and thus keep our bodies warm. Contrarily, in summers, white clothes are preferred as they absorb very little and reflect more of it, offering us a sense of cool.

Q. 9. Why do water supply pipes in homes burst during severe cold?

A. In winters, the temperature can go below 0 degree C, the freezing point of water. When water freezes into ice, it expands and in the process, exerts a large force on the internal walls of the pipe, resulting in the bursting of water pipes.

Q. 10. How does a parachute help a person to descend safely in an emergency?

A. A parachute is made from a fabric having limited air permeability and has a large frontal area (diameter of 30 feet). Therefore, when it falls through the air, it faces great resistance from the air. The lift and drag forces due to air flow balance the parachutist’s weight so that he comes down at a slow, constant speed of about 5 m/s.

Q. 11. A lighted candle is extinguished when we cover it with a tumbler. Why?

A. The process of burning requires oxygen and when a lighted candle is covered with a tumbler, the oxygen in the air inside is consumed by it. The inside of the tumbler becomes full of the products of combustion e.g. carbon dioxide, which do not support combustion, thereby extinguishing the candle.

Q. 12. Why does a cyclist bend inwards while taking a turn around a corner?

A. By bending inwards, the cyclist throws a part of his weight inwards to produce the centripetal force necessary to negotiate the curve.

Q. 13. Why does ice float on water while it sinks in alcohol?

A. Water has more density than alcohol. So the upward thrust (buoyant force) produced when the ice cube is placed in alcohol is not enough to support its weight, leading to its sinking.

Q. 15. Cooking food on the hills is difficult. Why so?

A. In hilly areas, the atmospheric pressure is low. Consequently, water boils at a lower temperature in such areas, resulting in slower cooking.

Q. 16. Why is the air removed from inside the electric lamp?

A. This is done to avoid the oxidation of the filament inside.

Q. 18. Why does a thick glass tumbler crack more easily than a thin one when we pour hot water into it?

A. When we pour hot water into a glass tumbler, its inner surface starts expanding outwards. Since glass is a poor heat conductor, the heat from the hot water is transmitted very slowly to the outer surface. It does not get as hot as the inner surface and expands outwards to a lesser extent. This unequal expansion, with the inner surface expanding against the outer surface, causes the cracking of the glass tumbler.

Q. 19. Why is the miner’s safety lamp considered to be a safe lamp?

A. The flame is covered with a metal gauge, which drives away heat and the ignition temperature of the inflammable gases inside is not reached.

Q. 20. Why does a pendulum clock, which keeps right time in winters, go slow in summers?

A. The time period of a pendulum clock is directly proportional to the pendulum length. During summers, the length of the pendulum increases and hence, its time period also increases.

Q. 21. In what way is a clinical thermometer different from an ordinary thermometer?

A. The range of a clinical thermometer is between 95 F - 110 F. Besides, there is a tiny constriction in the mercury thread, due to which it can be taken out of the mouth safely and the reading taken easily.


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