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

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
the full weight of the barrel comes into play when it is pulled

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Physics
A white and smooth surface is

Bad absorber and good reflector of heat
Bad absorber and bad reflector of heat
Good absorber and bad reflector of heat
Good absorber and good reflector of heat

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