Physics
Railway tracks are banked on curves so that

Necessary centrifugal force may be obtained from the horizontal component of the weight of the train
Necessary centripetal force may be obtained from the horizontal component of the weight of the train
The train may not fall down inwards
No frictional force may be produced between the tracks and the wheels of the train

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Physics
Stars twinkle because

the intensity of light emitted by them changes with time.
the distance of the stars from the earth changes with time.
None of these
the refractive index of the different layers of the earth's atmosphere ch

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