Scientific Reasons From Past Papers 13

➽ Why does ice not melt readily when salt is sprinkled over it?
Answer: When salt is sprinkled over ice, some of it dissolves. As dissolution of the salt is accompanied by absorption oh heat. The temperature of the system falls below 0 degree Celsius. Hence, ice does not melt readily.

➽ It is difficult for firemen to hold a hose, which ejects large amount of water at a high velocity. Why?
Answer: the water which comes out of the fireman’s hose carries large momentum as its velocity is very high. The equal and opposite reaction force pushes the fireman backwards with a great speed satisfying the law of conservation of momentum. As a result, it is difficult for the fireman to hold the hose.

➽ In the outer space astronauts talk to each other through radios. Why?
Answer: sound waves need a material medium for its propagation. There is no air in space and hence, sound waves cannot travel. However, radio waves can travel through space. The astronauts can see each other because light, like radio waves, can travel through vacuum.

➽ Sonar scanners are used by doctors. Explain.
Answer: sonar scanners send out ultrasonic waves, which are reflected by body tissue and organs. From the pattern of reflections, a computer can build up an image of the internal structure which is vital for diagnosis.

➽ Why the flash of lightning is seen before the sound of thunder is heard?
Answer: the velocity of light is much greater than that of the sound. Therefore, flash of lightning is seen before the sound of thunder is heard.

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