Running shoes have grips on their soles to prevent the wearer from slipping over: an example of "useful" friction. When two objects which are touching try to move past each other, they rub against each other. This produces a force called friction which tries to stop the movement. If the surfaces of the objects are rough, like sandpaper, then friction is greater. If they are smooth and slippery, especially if lubricated with oil or grease, then friction is less.
Friction in machines
Friction is “the enemy of machines”: it opposes movement, causes wear and tear and changes useful energy into waste heat. But friction can also be helpful. A vehicle slows down suddenly because of friction provided by its brakes. A bulldozer’s tracks dig into the earth with so much friction that they cannot slip. So the bulldozer can push huge mounds of soil.
Friction saves the Earth from catastrophe on a daily basis. Friction between meteoroids and the air increases their temperature and ignites them, so they burn up safely in the Earth's atmosphere as "shooting stars".
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