100 Important Scientific Reasons: Firemen to Hold a Hose and Outer Space Astronauts Talk

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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 formed 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 flow 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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