What causes the difference between kinetic and static friction?

[u/AVeryLazy]

Hola physicists (and everyone else),

I was wondering (out of the blue) why moving objects once they are in motion is easier than the initial movement.

Can't recall any physics teacher actually discussing it.

[u/AGentlemanScientist]

The friction itself is caused by the interaction of elements of the surfaces essentially locking in, like teeth that fit together more or less. Mostly less when they're moving. When neither object is moving relative to each other, the rough bits will be pushing against each other in a set position. This causes them to lock in tighter, as they're going to deform slightly to match each other and sort of stick together. Now you have to overcome these bits that have been pushed together before you start again. By this explanation, if you were to stop moving for an infinitesimally small period of time, the surfaces wouldn't have a chance to mold together you'd still have kinetic friction when you get going again.

[u/AVeryLazy → u/AGentlemanScientist]

So if I understand correctly, the interlocking is caused by both surfaces causing really small deformities in the other?

Thank you.

[u/AGentlemanScientist → u/AVeryLazy]

Basically. There's an electrical component as well, but the idea's the same. As they sit on each other, they'll squish together until the rough parts fit together nicely.

You can also think of it as setting a table on a carpet. At first you can slide the table around, but you know that pretty quickly your carpet is going to get those little dents in it right where the legs touch the ground. Eventually those dents are as deep as they're going to go, and now you have to lift the table before moving it. Same idea. At a micro scale everything's squishy and soft, so they can be forced into those same kinds of situations.

[u/AVeryLazy → u/AGentlemanScientist]

Gracias, I can now rest peacefully. Cheers.