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Intuition on static and kinetic friction comparisons

Video transcript
I mentioned in the last several videos that the coefficient of kinetic friction tends to be less, sometimes it'll be roughly equal to, the coefficient of static friction. But this might lead you to-- at least, a question that I've had in my mind, and I still have to some degree-- is why? Why is the coefficient of kinetic friction lower? Or why can it be lower? And the current best theory-- one I can visualize in my head, and based on the reading that I've done-- is the difference between-- so let's think about it this way. So if we look at it at a kind of a regular human level, maybe we have a block. So this is the static case. So let's think about the static case. Let me draw it like this. So I'll draw the static case over here. So I have a block that is stationary on top of-- let me do the surface in a different color-- on top of some type of surface right over here. And over here, I'm going to have a block moving at a constant velocity relative to some surface, relative to the same surface. And so let me draw it out. So this is moving at some constant velocity. And so the interesting thing here is, assuming that these are the same masses, that these are the same surfaces, is, why should the coefficient of friction here-- why should the coefficient of static friction-- so here, since this is stationary, what's under play is the coefficient of static friction. Why should that be larger than the coefficient of kinetic friction over here? Why should that be large than the coefficient of kinetic friction? Or another way to think about it is, you would need to apply more force to overcome the static friction here, and start to get this accelerating, than you would need to apply to get this already moving body to accelerate. Because there would be kind of a less of a responsive friction force. So let's think about that a little bit. So what I'm going to do is zoom in into the atomic level. And so when you zoom in to the atomic level, almost nothing is completely smooth. So the surface over here might look something like this. So I'm going to draw the molecules that make up the surface, the best to my ability. So the molecules, when you zoom up really close for the surface, might look something like this. So we're really zooming into the atomic level, unimaginably small level. Much smaller than that box I just drew. But I'm just trying to look at what's happening with the atoms where they contact, or the molecules where they contact? And the box's molecules might look something like this. They aren't completely smooth. And hopefully this video also emphasizes that all of these forces and all of this contact that we're talking about in these videos-- and it's actually interesting philosophically-- nothing is ever really in contact with each other. You really just have atoms that are repulsing each other, because their electrons the electromagnetic force of repulsion between them is not allowing them to get any closer together. So that's all-- when you push something, it's just the electrons in your hand pushing on the electron-- or the electronic clouds in your hand pushing on the electron clouds of, say, the pen you're holding, or the key on your keyboard, or the mug, so that it repulses it and causes it to go in the other direction. So there's never any of this thing like, what we imagine in our heads, real contact. And if you really want to blow your mind-- and watch the chemistry videos if you want understand this-- is that most of these atoms are actually free spaced themselves. That the electron cloud-- or I guess where most of the probability of finding the electron-- is huge compared to the size of the electron, or the size of the nucleus. So it's kind of just a lot of free space pushing on a lot of other free space through the electromagnetic force. But anyway, we're talking about friction here. So if you were to really zoom in here, when this thing is stationary, the surfaces aren't actually even. And so you could imagine that these molecules that you have, sometimes when it's sitting stationary, they might be kind of fit into each other. They've kind of slid in to maybe these little ruts here and there. And so if you're trying to move this object, if you're trying to accelerate it to the left with some force, you have to overcome, essentially, either-- for example, this part right over here either has to somehow break off, or the whole thing has to be shifted up a couple of atoms or a couple molecules. Or maybe, this part over here has to be broken off, or has to be shifted down one atom-- you wouldn't notice these things. You wouldn't notice the shifting of a block, or the shifting of the floor. You wouldn't notice it by the width of a molecule, or diameter of an atom or molecule. But that's essentially what you're going to have to do. Or you have to rip them off entirely in order to start this thing moving. Once something is already moving-- and this is at least how I think about it-- it doesn't have a chance to settle into these little ruts. So let me draw something that's already moving. And I'll try to draw a similar surface. So I'm trying to draw the surface that looks, essentially, just like the one I drew. So maybe it looks like that. This is supposed to be the same surface. But once it's moving, it's not sitting in these ruts anymore. The whole thing is moving. So it's kind of sliding across the top. And so now it might look something like this. I'll try my best to draw it. Maybe this has been shifted up a little bit so that it could start sliding. You've overcame the static friction. So now it is-- I'm trying to draw the same surface here, give or take-- so now it's moving. It doesn't have a chance to really settle in. It has to kind of bounce along the top. And so that's the best understanding. And so the real force of the friction here, as it's moving along it still might every now and then bounce into a little ruts here and there. But you also have any type of chemical bonds that form between the atoms temporarily that keep breaking and forming. And in order to keep this thing, I guess, moving, or especially if you want to accelerate it, you're going to have to keep breaking these bonds. And so that's essentially the force of friction that you're overcoming. Here you might have those same bonds. And not only do you have the same bonds, but you also have to overcome these ruts, or these little ragged parts that have a time to settle in to these little nooks that you have to overcome even more. So that's the intuition. And you know this is actually still an area of research. So it's not like this cut and dry thing. And it's a fun thing to think about what's happening at the atomic level. But this is the general intuition of why the coefficient of static friction is higher than the coefficient of kinetic friction.