Physics
When a bullet is fired from a gun

The gun does not move
The gun recoils with the same momentum as the bullet
The gun moves forward
The gun jumps up

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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
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

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