31) Cloudy nights are usually warmer than the clear ones.
Ans: The clouds serve as a barrier and prevent the loss of heat. Thus the warm temperature is maintained.
32) Why do some people snore?
Ans: Some people snore during their sleep because the breathing action produces sound. This is so because the pharynx or the windpipe offers resistance to the air taken in or expelled.
33) Why do we sometimes sleep walk?
Ans: Sleep walk is a sort of disorder of sleep in which a person starts walking during his sleepThe sleeper walks and performs complex activities automatically without regaining consciousness.
34) Climbers bend forward while climbing mountain.
Ans: While climbing a mountain, a climber bends his body forward in order to keep the centre of gravity of his body within two thirds portion. According to this principle the Pisa Tower is not falling.
35) Why climbers get their food by climbing on other trees?
Ans: The climbers are mostly parasites and they cannot manufacture their food. Therefore, they climb on other trees and get food from them.
36) Mars is called red planet.
Ans: Mars is called the red planet because its colour is red due to the desert like surface.
37) Vitamin D is the essential component of the body.
Ans: Vitamin D is necessary for body because it is essential for bone formation and retention of calcium in the human body. Vitamin D also protects the teeth.
38) The weight of the object is less at the equator than at the poles.
Ans: The weight of an object is the product of mass and force of gravity. The equatorial diameter is more than the polar diameter. Thus, the force of gravity is more at the poles and the weight is more there.
39) The dogs pant, the birds open their mouth and the elephants move rapidly their ears.
Ans: All of them do not sweat; they use the above mentioned mechanisms to cool themselves.
40) A geostationary satellite appears standstill to a viewer on the equator of earth.
Ans: Because the distance remains the same.
41) Why do the stars twinkle?
Ans: The light from the stars travels through different layers of space of varying densities. Therefore, the light rays deviate from their original path. Further, these layers are not stationary but keep on moving. This leads to the twinkling of stars.
42) On what days do we have equal days and nights all over the world? And why?
Ans: On 23 September and 21 March we have equal days and nights all over the world. This is so because on these days, the rays of the sun fall vertically on the equator at noon. Both the poles receive equal rays of the sun. As a result, exactly one-half of each hemisphere receives the sun’s rays. This makes day and night equal.
43) Every fourth year has 366 days. Explain.
Ans: One revolution of the earth around the sun takes 356 days and 6 hours. But we consider a year as consisting of only 365 days and ignore 6 hours. In four years the difference becomes as much as 24 hours or one day. Hence, to every fourth year we add one day. That year of 366 days is known as leap year.
44) Why are igneous rocks called primary or parent rocks?
Ans: Igneous rocks are formed when the molten material from volcanoes gets solidified. This material is liquid, hot and sticky which moves towards the surface through cracks and joints. All other rocks derived from these rocks. Hence, they are called primary or parent rocks.
45) Why are the areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere very cold?
Ans: These areas are very cold because the sun does not rise much above the horizon. Therefore, its rays are always very slanting which emit minimum heat. These areas being very cold are called Frigid Zones.
46) Why are the three hot regions of the world-equatorial forests, savannah lands and hot deserts-not found in Europe?
Ans: The three hot regions of the world-equatorial forests, savannah lands and hot deserts-are not found in Europe because Europe is the only inhabited continent situated entirely outside the tropics.
47) Why is the lowest layer of the atmosphere in contact with the earth’s surface, the warmest?
Ans: The lowest layer of the atmosphere in contact with the earth’s surface is the warmest because the atmosphere is heated mainly from the below. 48) Why does the temperature above the ocean and land masses vary even on the same latitude?
Ans: Temperature above the oceans and land masses varies even on the same latitude because of the differential heating of land and water, i.e., land mass is heated and cooled more rapidly and to a greater degree than water. 49) Why are marine animals able to live at great depths than marine plants?
Ans: As marine animals do not depend upon sunlight for their survival, they are able to live at great depths where there is permanent darkness.
50) Why do trees of coniferous forests possess needle-like leaves?
Ans: The needle-like leaves limit transpiration and thus enable conifers to grow in the drier areas.
51) Why do equatorial forests appear evergreen?
Ans: In equatorial forests trees often shed a few leaves or shed their leaves seasonally, but most of the trees retain their leaves for most of the time so that the forests appear evergreen.
52) Why is petroleum often called black gold?
Ans: Petroleum is often called black gold because of its great demand in the modern industry and for domestic use. It provides fuel for heat and lighting, lubricant for machinery and raw material for a number of industries.
53) Why are the kangaroos called marsupials?
Ans: Marsupial means broad-pouch. Since kangaroos have a pouch-like fold of skin near the stomach in which they carry their young ones, they are called marsupials.
54) Why is platypus considered a strange animal?
Ans: Platypus is a strange animal because it is an animal-bird that survives under water, walks on the ground and digs tunnel under the ground. It is a four-legged animal that lays eggs like a bird. It is found in Australia.
55) Why is a person in moving vehicle thrown forward when the vehicle stops suddenly?
Ans: A person in a moving vehicle is in a state of motion. When the vehicle suddenly stops his body tends to remain in a state of motion due to inertia and he is thrown forward.
56) Earth is continuously pulling moon towards its centre. Why does not the moon fall on to the earth?
Ans: it is so because the gravitational attraction of the earth provides the necessary centripetal force to the moon for its orbital motion around the earth due to which the moon is revolving around the earth.
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