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Current time:0:00Total duration:7:21

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 that I won 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 on a off we look at it at a kind of a regular human level maybe we have a block maybe we have a block so this is the static case so let's think about the static case over 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 me to 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 some at a constant velocity 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 constant 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 this is 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 larger than the coefficient of kinetic friction or another way to think about it is you would need to apply you would need to apply more force to overcome the static friction here and start to get this accelerating then you would need to apply to get this already moving body to accelerate because there would be kind of a less of a 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 into the atomic level and so when you zoom into the atomic level almost nothing is completely smooth so the surface over here the surface over here might look something like this so I'm gonna draw it I'm gonna draw the molecules that make up the surface so the best to my ability so the molecules when you zoom up really close for the surface might look something like this and when you so you know we're really we're really zooming into the atomic level unimaginably small level much smaller than that box that is true but I'm just trying to look at what's happening with the atoms where they contact are the molecules where they contact and the boxes molecules might look something like this the boxes molecules might look something like this they aren't completely smooth they aren't completely smooth and hopefully this video also emphasizes that all of these forces and all 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 and that are the dult that are repulsing each other because there are electrons they are 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 it's just the electrons in your hand pushing on the electron or the electron clouds in your hand pushing on the electron clouds of say your the pen you're holding or the key on your keyboard or the mug so that it so that it repulses and causes it to go in the other direction so that you there's no ever there's never any this thing like real what we imagine our heads real contact and if you really want to blow your mind and watch the chemistry videos if you want to understand this is that most of these atoms are actually free space themselves that the electron cloud is huge relative 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 you have the surfaces aren't actually even and so you can imagine that these molecules that you have sometimes when the sitting stationary they might be kind of fit into each other they've kind slided slid into 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 of 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 we wouldn't notice the shifting of a block or the shifting of the floor you wouldn't notice it by the 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 a lot of it hasn't it has it's it doesn't have a chance to settle into these little ruts so let me draw something that's already moving and I'll draw the same 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 there's supposed to be the same surface but once it's moving it's kind of over it can't 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'm trying my best to draw it so now it might look something like this maybe this has been shifted up a little bit so that it can starts lying you've overcome the you've over overcame the static friction so now it is so now it is let me draw the same trying to draw the same surface here so 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 the real the real force of the friction here there will be still some you know as it's moving along it still might every now and then bounce into little ruts here and there but you also have 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 in order to keep this thing for I guess moving or if 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 those same bonds but you also have to overcome these ruts or these little you know these little ragged parts that that are that have a time to settle in to these little looks that you have to overcome even more so that's the intuition you know this is actually still an area of research so it's not like this you know 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