What is friction?

Dave Consiglio:

Very short answer: It is the force that resists motion.

Medium length answer: All things have “roughness.” Friction is the force exerted by that roughness as an object passes by another, acting in the direction opposite to the motion.

Long answer: Atoms have electron clouds. The way I like to think about this is that atoms are like a black garbage bag full of very angry bees. You can’t know exactly where the bees are, but you know how many there are, and you know they’re more or less within the volume defined by the bag. Oh, and you know they’re angrily buzzing around.

So as two atoms pass one another, the farthest reaches of their electron clouds interact. Since electrons are negative, the bags repel one another, deforming and pressing back against the motion.

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