Physics
Solids have definite shapes.This is because

The forces of cohesion of the molecules within are very much less in them
The forces of adhesion are less
The forces of adhesion are large
The forces of cohesion are very large

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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
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

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