Physics
It is easier to roll a barrel full of coal tar than to pull it because

more surface area of the barrel is in contact with the road when it is pulled
coal tar is a liquid and it flows in the barrel when it is rolled
the full weight of the barrel comes into play when it is pulled
the rolling friction is much less than the sliding friction

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Physics
RADAR is used for:

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

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