Physics
Railway tracks are banked on curves so that

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

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Physics
When a piece of pure silicon is doped with aluminium, then

The conductivity of the doped silicon piece will remain the same
The doped silicon piece will become p-type
The resistivity of the doped silicon piece will increase
The doped silicon piece will become n-type

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