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

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Physics
A dynamo is a device which

Creates electrical energy
Converts mechanical energy into electrical energy
Creates mechanical energy
Converts electrical energy into mechanical energy

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