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
The blue colour of sky is due to

Refraction of sunlight
Reflection of sunlight
Scattering of shorter wavelengths of sunlight in the atmosphere
Scattering of longer wavelengths of sunlight in the atmosphere

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