Physics
Railway tracks are banked on curves

necessary centrifugal force may be obtained from the horizontal component weight of the train
the train may not fly off in the opposite direction
necessary centripetal force may be obtained from the horizontal component of the weight of the train
to avoid frictional force between the tracks and wheels

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Physics
The wall of a dam is broader at the base

Because streamlining is required
To withstand pressure that increases in a horizontal plane
To withstand pressure that increases with depth
To withstand pressure that is increased with atmospheric pressure

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