Physics
The velocity of falling rain drops attains a terminal value because of

Viscous force exerted by air
Surface tension
Air current
Upward thrust due to air

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Physics
Skating on ice is possible because

The pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water
Ice is cold
The surface of ice is smooth
The pressure on the ice due to skate raises its melting point

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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
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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