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

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

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Physics
Thermohaline circulation is mainly due to

Cold, dense ocean water sinking and warmer water replacing it
Winds causing circulation of ocean water
Warmer, less dense water sinking in the ocean and cold water replacing it
Warm water being added to the ocean due to industial waste

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