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Intuition on static and kinetic friction comparisons

AP.PHYS:
INT‑3.B.2.1 (LO)
Why static friction is harder to overcome than kinetic friction. Created by Sal Khan.

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  • blobby green style avatar for user chwang733
    In several videos before, you mentioned that a block of ice on ice does not cause a lot of friction. But in the chemistry playlist, water molecules form hydrogen bonds, which take a lot of energy to break. Wouldn't the block of ice slow down even faster?
    (72 votes)
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  • male robot hal style avatar for user Conor McKenzie
    So, based on Sal's logic, as the speed of the block got higher and higher, the number of nooks it would get stuck in would approach 0, and the chemical bonds made would get farther between and briefer, to the point where the block passes by the other surfaces' atoms so quickly that there is not enough time for any bonds to be made and held for a significant time. Would that not mean that the force of kinetic friction (1) approaches 0 as the speed approaches infinity, therfore (2) is not constant, and thus (3) should be given for different speeds of an object or in an equation relating the coefficient of friction to the speed of the object?
    (27 votes)
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  • blobby green style avatar for user ViewSk8
    Why do some objects have higher coefficients of friction than others?
    (12 votes)
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    • starky ultimate style avatar for user Soul Seeker
      Try touching different surfaces like polished steel, sandpaper, wood etc. You will fell a few of the surfaces are smoother than others. This is because the rougher surfaces have more tiny projection as compared to the smooth surfaces. Therefore, affecting the coeff. of friction.
      (33 votes)
  • blobby green style avatar for user pagedout
    Your videos are very interesting but I am unsure on your explanation here.
    You state that the theory is it settles and this causes it to take more force to move it. However, settling would be a function of time in one spot (as implied by jumping over ruts comment) so this would seem to imply that the Mk should vary based on how fast an object is moving (less time to settle) which is not part of your equation. Simple experiments also contradict. So, what makes you think of it like this?
    Thank
    (10 votes)
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  • female robot ada style avatar for user chinmayajoisa
    Won't the coefficient of static friction be greater than the coefficient of kinetic friction In all cases, just because additional force is required to override the inertia of rest possessed by the body?
    (8 votes)
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  • orange juice squid orange style avatar for user Kai
    Is there anything that has 0 friction? If not is it possible? What material do we know of today that has the lowest friction?
    (4 votes)
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    • leaf grey style avatar for user patoof
      Objects in space have no friction whatsoever. This is why if you push an object in space, it will continue moving in the same line forever - unless acted on by external force, such as gravity.
      Newton's 1st law states: An object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
      (7 votes)
  • female robot ada style avatar for user Chang Ho Lee
    I have a semi-related question:
    Let's say that you have a surface and an object, and you slam down that object with a certain amount of force. Let's also say that theoretically, right before the two surfaces make contact, two electrons from different atoms of different surfaces line up together. So, when the two surfaces "make contact", is it possible for the two electrons to actually touch each other?
    (2 votes)
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    • male robot hal style avatar for user Andrew M
      It is possible for electrons to "collide" if they are given enough energy. It is not the case that their opposite charge would have to stop them from colliding, just as magnets can be forced to have their north poles touch one another.

      The real problem with thinking about collisions is that electrons aren't really particles. We often think of them as little particles, and sometimes we think of them as waves, depending on what experiment we are trying to do. When you accelerate electrons to speeds that allow them to collide, you can't really describe that collision as if they were two bullets slamming into each other. You have to start to use quantum mechanics to describe what will happen. It gets very weird.
      (6 votes)
  • aqualine ultimate style avatar for user Riido
    Would the kinetic friction decrease as the speed of the object increases? The moleculs would have even less time to form bonds and have an easier time bouncing and avoiding those valleys.
    (5 votes)
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  • female robot ada style avatar for user Chang Ho Lee
    i don't get how bonds between the two molecules form. I mean, if the electrons are repulsing both atoms away from each other, then how is it possible to create bonds if they never make contact?
    (5 votes)
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  • aqualine sapling style avatar for user sahaja mahanty
    isnt there any friction between atoms?
    (3 votes)
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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.