100 Important Scientific Reasons Page 3 of 4
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.
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.
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.
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.
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.
65)
In the outer space astronauts talk to each other
through radios. Why?
Ans: 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.
66)
Sonar scanners are used by doctors. Explain.
Ans: 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.
67) Why the flash of lightning is seen before the
sound of thunder is heard?
Ans: 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.
68) Bats have poor eyesight but are able to home
their prey with great accuracy. Also, dolphins can avoid fishing nets and can
detect fish at night. Explain
Ans: Bats emit
high frequency (1, 20,000 Hz) sound waves and listen with their sensitive ears
for any echoes. From the time taken to hear the echo and from the nature of
sound received. Bats are able to estimate the distance and the type of
surroundings. This process is known as echolocation. Same process is used by
the dolphins.
69) When
low flying aircraft passes overhead, we sometimes notice a slight disturbance
on our TV screen. Why is it so?
Ans: a low flying
aircraft reflects TV signal. Due to the interference between the direct signal
and the reflected signal, there is disturbance on the TV screen.
70) In automobiles, why are convex mirrors used to
see the traffic from behind?
Ans: Convex
mirrors are used in automobiles because they form erect and diminished images
of the object. Thus, it helps the drivers to get a wider field of view of the
traffic coming from behind.
71) Why is mirage formes in the deserts?
Ans: A mirage is
formed owing to total internal reflection. To the observer at a distance, the
reflected image of the object appears behind the reflecting surface, as if the
object were in front of it, but actually it is just an illusion.
72) What will be the colour of grass in blue light?
Ans: Grass will
appear blackish in colour in blue light because it has the property of
absorbing all other colours except its own colour. The blue rays falling on
grass will be absorbed by it, and it will appear dark coloured.
73) A bird perches on a bare high power line and
nothing happens to it. A man standing on
the ground touches the same line and gets a fatal shock. Why?
Ans: When bird
perches on a live high power line, no current passes through the body of bird
because there is no potential difference between live wire and the body of bird
as the potential of bird’s body is the same as that of wire.
When a man
standing on the ground touches the same wire then due to a large potential
difference between his hands and feet, a large current flows through his body
to the earth.
74) During lightning it is safest to be inside a car
rather than under a tree or in the open. Give reason.
Ans: When a
person is in the open or under the tree, the lightning passes through his body
to the earth. On the other hand, for a person inside a car, the car provider
shielding and the electric field inside the car is zero, thus lightning does
not affect the person inside it. The lightning actually passes through the
metallic body of the car to the earth without affecting the person sitting
inside it.
75) Why is earthing desirable for electric
appliances?
Ans: Earthing
helps the current move into the earth in the event of short-circuit, without
giving a shock to the user.
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