57) Which of the two-glass or rubber is more elastic and why?

Ans: Glass is more elastic than rubber because for a given applied force per unit area, the strain produced in glass is much more than that produced in rubber.

58) Animal like camel can easily walk in the desert sand while other animals like donkeys, dogs and horses cannot. Explain.

Ans: The camel has very broad and large feet. As a result of the large surface area in contact with the ground, it exerts less pressure on the sand and sinks only slightly in it. Other animals have smaller feet which exert more pressure on the sand. As a result, they sink more in the sand and cannot walk easily in desert.

59) Why does a small quantity of liquid assume spherical form?

Ans: A small quantity of liquid assumes a spherical form due to surface tension which tends to reduce the surface area. A given mass will acquire minimum surface area if it assumes a spherical shape.


60) Why does an iron needle float on clean water but sink when some detergent is added to this water?

Ans: Due to surface tension, the free surface of liquid at rest behaves like a stretched membrane. When an iron needle floats on the surface of clean water, its weight is supported by the stretched membrane. When some detergent is added to this water, its surface tension decreases. As a result of
it, the stretched membrane on the surface of water is weakened and is not able to support the weight of needle. Hence the needle sinks in such water.

61) Why is cooking quicker in a pressure cooker?

Ans: The boiling point of water depends upon the pressure on its surface. Steam produced inside the cooker builds up pressure thereby raising the boiling point of water, which results in quick cooking.

62) Why does steam cause more severe burns than boiling water?

Ans: The amount of heat (latent heat) possessed by steam is much greater than the amount of heat possessed by water at the same temperature. Therefore, steam causes more severe burns than boiling water.

63) Why does ice not melt readily when salt is sprinkled over it?

Ans: 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.


64) It is difficult for firemen to hold a hose, which ejects large amount of water at a high velocity.Why?

Ans: 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.

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