Physics
RADAR is used for:

locating geostationary satellites.
receiving a signals in a radio receiver.
detecting and locating the position of objects such as Aeroplanes.
locating submerged submarines.

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to expansion in summer can be avoided
Air-gaps are necessary for bearing the weight of running train
Accidents due to contraction in winter can be avoided
Steel can be saved

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