Inclined planes and friction
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Inclined Plane Force Components
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Ice Accelerating Down an Incline
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Force of Friction Keeping the Block Stationary
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Correction to Force of Friction Keeping the Block Stationary
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Force of Friction Keeping Velocity Constant
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Intuition on Static and Kinetic Friction Comparisons
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Static and Kinetic Friction Example
Intuition on Static and Kinetic Friction Comparisons Why static friction is harder to overcome than kinetic friction
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- I mentioned in the last several videos
- that the coefficient of kinetic friction tends to be less
- sometimes little bit roughly equal to the coefficient of static friction
- but this might lead you to at least a question
- that I have had in my mind and I still have to some degree
- that why is the coefficient of kinetic friction lower or why can it be lower?
- And the current best theory that I
- can visualize in my head and based on the readings that I have done is the difference between--
- so let's think about it this way
- Look at a regular human level
- maybe we have a block, so this is the static case. Let me draw like this
- I will draw this static case over here
- So I have a block that is stationary on top of a surface in a different color
- On top of some type of surface right over here
- and over here I am going to have a block
- moving at some at a constant velocity relative to the same surface
- Let me draw out. So this is moving at some constant velocity
- and so the interesting thing here is assuming that these are the same masses
- These are the same surfaces
- is why should the coefficient of friction here
- why should the coefficient of static friction--
- so this is here, this is stationary what's under place of coefficient of static friction
- why should that be larger than the coefficient of kinetic friction over here?
- Another way to think about it, 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 less of a responsive friction force. So let's think about it little bit
- So what I am going to do is zoom in into the atomic level. So when you zoom into the
- atomic level, almost nothing is completely smooth
- So the surface over here might look something like this
- So, let me 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--you know we are really really zoom into the atomic level
- unimaginably small, much smaller than that boxes. But I am trying to look it
- what's happening with the atoms/molecules where they contact
- and the box's molecules look something like this
- They aren't completely smooth and hopefully this video also emphasizes
- that all of this forces and contacts we are talking about in this video --it's actually
- interesting philosophical, nothing is ever really in contact with each other. You really
- just have atoms and that are repulsing each other because they are 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 electron clouds
- in your hand pushing on the electron clouds of say the pen you are holding
- or the key on your keyboard or the mug
- so that it repulses and causes it to go in the other direction
- So there is never this thing like real what we imagine in our head 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 this atoms are actually free space themselves
- as the electron cloud is huge relative or I guess where most
- of the probability of finding the electrons 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
- lot of other free space through the electromagnetic force
- But anyway we are talking about friction here
- So if you would really zoom in here when this thing is stationary
- you have the surfaces aren't actually
- even and so you can imagine this molecules that you have sometimes when they sitting
- stationary, they might be kind of fit in to each other
- they kind of slit into these little ruts over there
- and so if you are trying to move this object, if you 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 things to be shifted up a couple of atoms or a couple of molecules
- or may be 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; you wouldn't notice it
- by the width of a molecule or the diameter of the molecule, but that's essential
- what you have to do or you have to rip them off entirely in order to start this thing moving.
- Once something's already moving, and this is at least I think about it
- a lot of--it doesn't have a chance to settle into these little ruts. So
- let me draw something that already moving and I will try to draw a similar surface
- So I am trying to draw the surface that looks essentially 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 the ruts anymore
- The whole thing is moving, so it's kind of sliding across the top
- So now it looks something like this. I will try my best to draw it
- Maybe this is been shifted up a little bit so that it can start slide
- you've overcome the static friction
- So now it is--something draw the same, I am 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
- So that's the best understanding and so the real force of the friction here
- there will be still you know as it's moving along it still might
- bounce into 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
- in order to keep this thing--if you want to accelerate it
- you are going to have to keep breaking these bonds
- So that's essentially the force of friction that you are overcoming
- Here you might have those same bonds and not only you have those same bonds
- but you also have to overcome these ruts or these little parts that have a time to settle in
- to little looks that you have to overcome even more. So that's the intuition
- you know this is actually still an area of research. It's not this cut and dry thing
- so fun thing to think about what's happening in the atomic level
- but this is the general intuition of why the coefficient of static friction
- is higher than the coefficient of kinetic friction
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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