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. Before the surfaces can move relative to each other, the bonds that cause this adhesion must be broken. In addition, the roughness of the surfaces means that at some locations, the asperities of one surface will settle into the valleys of the other surface — in other words, the surfaces will interlock.
These interlocked areas must be broken or plastically deformed before the surfaces can move. In other words, abrasion must occur. So, in most applications, static friction is caused by both adhesion and abrasion of the contacting surfaces.
Overcoming the static friction between two surfaces essentially removes both the molecular obstacles cold welding between asperities and, to some degree, the mechanical obstacles interference between the asperities and valleys of the surfaces to movement. Once movement is initiated, some abrasion continues to occur, but at a much lower level than during static friction. And the relative velocity between the surfaces provides insufficient time for additional cold welding to occur except in the case of extremely low velocity.
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