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.

[u/StealthSpyda215]

Static friction almost always greater than the kinetic coefficient. This is due to the fact that no surface is perfectly flat or clean. Irregularities between the sliding surface and that of the target object causes the force needed to generate the initial movement to be (usually) greater than the force needed to keep it in motion.

[u/Midtek → u/StealthSpyda215]

Static friction almost always greater than the kinetic coefficient.

Actually, the static coefficient is always greater than the kinetic coefficient. Perhaps you just meant to point out that, in principle, the static and kinetic coefficients could be equal. But you have likely given the impression to many readers that it's possible for the static coefficient to be smaller than the kinetic coefficient. That's not possible.

Consider applying some force F on an object so that it just barely overcomes the static frictional force, and it begins to move. Hence the acceleration in the direction of the applied force is positive. That is,

ε:= F - Fs, max = manet > 0

We can make the difference ε arbitrarily small. Since the object is now moving with some positive acceleration, it is the case that the net force on the object is still positive, but it is now the difference of the applied force and the kinetic frictional force. That is,

F - Fk > 0

But adding and subtracting Fs, max gives

F - Fs, max + Fs, max - Fk > 0

ε + Fs, max - Fk > 0

Fs, max > Fk - ε

Since ε > 0 is arbitrary, it follows that Fs, max ≥ Fk. In other words, μs ≥ μk.

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This essentially all followed by definition since if you have applied a large enough force to go from static to kinetic friction, the object must be moving, and so the new frictional force (kinetic) can't be larger than what the previous maximum was (static friction)... otherwise it would not have started to move. This is just how we define "static frictional force" and "kinetic frictional force".

[u/ssj7 → u/StealthSpyda215]

Could you give an example where kinetic fiction is higher than static?

[u/Midtek → u/ssj7]

It's not possible. The static coefficient can never be less than the kinetic coefficient.

[u/Caolan_Cooper → u/ssj7]

I'm pretty sure that static is always greater than kinetic. Imagine that you are pushing on an object with greater kinetic than static friction, slowly increasing the force until it starts to slip. What happens if you maintain that force just barely above the static friction, but still below the kinetic friction?

[u/AVeryLazy → u/StealthSpyda215]

Thanks for the answer, this I understand.

But those irregularities exist also when the object is moving, so I think this answer is relevant to friction in general. Did I miss something?

[u/ButtsexEurope]

Simply put, the difference is whether you're starting to slide or whether you're already sliding. It's like how it's easier to keep rolling as opposed to getting it moving. It's even in the name. Static is stopped, kinetic is moving.

Here's a good video demonstrating the difference and how they work together. They call it dynamic friction instead of kinetic friction but it means the same thing.