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Old Saturday, January 15, 2011
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Second part..........!!!

31. Question: Why are water or mercury droplets always round when dropped on a clean glass?

Answer: The surface of a liquid is the seat of a special force as a result of which molecules on the surface are bound together to form something like a stretched membrane. They tend to compress the molecules below to the smallest possible volume, which causes the drop to take a round shape as for a given mass he sphere has minimum volume.

32. Question: Why does a balloon filled with hydrogen rise in the air?

Answer: Weight of hydrogen is less than the weight of air displaced by it. In balloons hydrogen is normally filled because it is lighter than air.

33. Question: Why do we lean forward while climbing a hill?

Answer: In order to keeps the vertical line passing through our centre of gravity always between our feet, which is essential to attain equilibrium or stability.

34. Question: Why does smoke curl up in the air?

Answer: Smoke contains hot gases which being lighter in weight, follows a curved path because of the eddy currents that are set up in the air.

35. Question: Why does an electric bulb explode when it is broken?

Answer: The bulb encompasses partial vacuum and as it breaks, air rushes in causing a small explosion.

36. Question: Why does a man fall forward when he jumps out of a running train or bus?

Answer: He is in motion while in the train or bus. When he jumps out, his feet comes to rest while touching the ground but his upper portion which is still in motion propels him forward.

37. Question: Why does an ordinary glass tumbler crack when very hot tea or milk is poured in it?

Answer: When a hot liquid is poured into a tumbler, the inner layer of the tumbler gets heated, it expands before the outer layer and an unequal expansion of both layers causes the tumbler to crack.

38. Question: Why is a compass used as an indicator of direction?

Answer: The magnetic needles of a compass under the influence f the earth?s magnetic field lie in a north-south direction. Hence, we can identify direction.

39. Question: Why is water from a hand pump warm in winter and cold in summer?

Answer: In winter, the outside temperature is lower than that of water flowing out of the pump, and therefore, the water is warm. Whereas in summer, the outside temperature is higher than the water of the pump, and therefore, it feels cold.

40. Question: Why is a rainbow seen after a shower?

Answer: After a shower, the clouds containing water droplets act like a prism through which the white light is dispersed producing a spectrum.

41. Question: Why does a swimming pool appear less deep than is actually is?

Answer: The rays of light coming from the bottom of the pool pass from a denser medium (water) to a rarer medium (air) and are refracted (bend away from the normal). When the rays return to the surface, they form an image of the bottom of the pool at a point, which is little above the real position.

42. Question: Why is one?s breath visible in winter but not in summer?

Answer: In winter, water vapor contained in the breath condenses into small droplets, which become visible but in summer they are quickly evaporated and not seen.

43. Question: Why doesn?t the electric filament in an electric bulb burn up?

Answer: Firstly, because is made of tungsten which has a very high melting point (34100C) whereas the temperature of the filament required to glow is only 2700oC. Secondly, oxygen is absent since the bulb is filled with an inert gas which does not help in burning.

44. Question: Why does blotting paper absorb ink?

Answer: Blotting paper has fine pores, which act like capillaries. When a portion of blotting paper is brought in contact with ink, ink enters the pores due to surface tension (capillary action f liquids) and is absorbed.

45. Question: Why does a small iron sink in water but a large ship float?

Answer: The weight of water displaced by an iron ball is less than its own weight, whereas water displaced by the immersed portion of a ship is equal to its weight (Archimedes? Principle).

46. Question: Why does ice float on water?

Answer: The weight of the ice block is equal to the weight of the liquid displaced by the immersed portion of the ice.

47. Question: Why does moisture gather outside a tumbler containing cold water?

Answer: The water vapour in the air condenses on cooling and appears as droplets of water.

48. Question: Why does kerosene float on water?

Answer: Because the density of kerosene is less than that of water. For the same reason cream rises in milk and floats at the top.

49. Question: Why is the water in an open pond cool even on a hot summer day?

Answer: As the water evaporates from the open surface of a pond, heat is taken away in the process, leaving the surface cool.

50. Question: Why is it less difficult to cook rice or potatoes at higher altitudes?

Answer: Atmospheric pressure at higher altitudes is low and boils water below 100C. The boiling point of water is directly proportional to the pressure on its surface.

51. Question: Why is it difficult to breathe at higher altitudes?

Answer: Because of low air pressure at higher altitudes the quantity of air is less, and so that of oxygen.

52. Question: Why are winter nights and summer nights warmer during cloudy weather than when the sky is clear?

Answer: Clouds being bad conductors of heat do not permit radiation of heat from land to escape into the sky. As this heat remains in the atmosphere, the cloudy nights are warmer.

53. Question: Why is a metal tyre heated before it is fixed on wooden wheels?

Answer: On heating, the metal tyre expands by which its circumference also increases. This makes fixing the wheel easier and therefore cooling down shrinks it; thus fixing the tyre tightly.

54. Question: Why is it easier to swim in the sea than in a river?

Answer: The density of sea water is higher; hence the up thrust is more than that of river water.

55. Question: Who will possibly learn swimming faster-a fat person or a thin person?

Answer: The fat person displaces more water which will help him float much more freely compared to a thin person.

56. Question: Why is a flash of lightening seen before thunder?

Answer: Because light travels faster than sound, it reaches the earth before the sound of thunder.

57. Question: Why cannot a petrol fire be extinguished by water?

Answer: Water, which is heavier than petrol, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire. The latter is true if a small quantity of water is poured.

58. Question: Why does water remain cold in an earthen pot?

Answer: There are pores in an earthen pot which allow water to percolate to the outer surface. Here evaporation of water takes place thereby producing a cooling effect.

59. Question: Why do we place a wet cloth on the forehead of a patient suffering from high temperature?

Answer: Because of body?s temperature, water evaporating from the wet cloth produces a cooling effect and brings the temperature down.

60. Question: When a needle is placed on a small piece of blotting paper which is place on the surface of clean water, the blotting paper sinks after a few minutes but the needle floats. However, in a soap solution the needle sinks. Why?

Answer: The surface tension of clean water being higher than that of a soap solution, it cans support the weight of a needle due to its surface tension. By addition of soap, the surface tension of water reduces, thereby resulting in the sinking of the needle.

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