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
more surface area of the barrel is in contact with the road when it is pulled
the rolling friction is much less than the sliding friction

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Physics
When a pail of water is swung in a vertical circle, the water does not fall out at the top of the loop when the speed is

Above a certain minimum value depending on amount of water in the pail
Below a certain minimum value depending on amount of water in the pail
Below a certain minimum value irrespective of amount of water in the pail
Above a certain minimum value irrespective of amount of water in the pail

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