Friction is a force between two surfaces that are sliding, or trying to slide, across each other. For example, when you try to push a book along the floor friction makes this difficult.
Friction always works in the direction opposite from the direction the object is moving, or trying to move. Friction always slows a moving object down.
The amount of friction depends on the materials from which the two surfaces are made. The rougher the surface, the more friction is produced.
For example, you would have to push a book harder to get it moving on a carpet than you would on a wooden floor. This is because there is more friction between the carpet and the book than there is between the wood and the book.
Friction also produces heat. If you rub your hands together quickly, you will feel them get warmer.
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