Intuition on static and kinetic friction comparisons | Physics | Khan Academy

Why static friction is harder to overcome than kinetic friction. Created by Sal Khan.

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[anushabenny]

Sal is just such an amazing teacher. this is exactly how pictured it in my head (it made more sense to me) but I wasn't sure if it was correct.

[krystalpaquette]

Thank you so much! I'm good with accepting the math, but not the theory. This helped me understand it a lot better.

[khanacademy]

@zelos88 More because the molecules can be locked into each others' nooks while they are stationary (although this might also increase the number of molecules that can interact with each other since they would have more of the surface at the atomic level exposed to each other)

[arbaces]

Thanks, as always, for the great videos.

I want to make just one comment on your video. At 1:56, you compare the force necessary to get the left block to accelerate versus that of the right block "... to get this already moving body to accelerate."

Note that the right block will accelerate even if you apply no force to it. The body will have friction directed opposite its motion. So honestly, you can apply zero force to the block at right to get it to accelerate.

(more continued)

[arbaces]

Since this video is focused on physical intution of why mu-static is greater than mu-kinetic, this is really just a side-note.

Thanks again :)

[Shain]

Does kinetic frictions depends on velocity? Based on this video u might assume that yes. Is there an average correlation between speed an kinetic? Of chemical bonds? Temperature? All these corelations might b very interesting in better understanding the issue....

[trainerMC]

Intuitively, I also might think that the mu-kinetic is smaller because for any given comparison, the temperature is higher in the system that is moving, and therefore will allow for easier intermolecular bond/interaction breaking. Just my thought, not sure if its legitimate.

[Lakrushma]

Help3d a lot for my physics 20 thx for the vid

[scottnstone]

This video contains an incorrect statement that at the molecular level there is a force of repulsion between the electron clouds of the molecules of the two surfaces. If that were true, then when you brought the surfaces into contact there would be a larger and larger repulsive force as the surfaces are brought closer. But we do not observe that. Instead, as Freeman Dyson has shown, the reason solids are relatively impenetrable is the quantum degeneracy pressure (aka Pauli exclusion principle) that prevents electrons from occupying the same quantum real estate. An electron cannot force its way into the orbitals occupied by electrons in the opposing material, in the same way you are not legally allowed to build your house in somebody else's back yard, even though there appears to be plenty of unoccupied space.

[adetolaraymondogunjimi]

I don't agree with the assumption that the two objects rough surfaces slide into one another increasing the coefficient of friction, and that when the two touching surfaces are in motion the surfaces do not slide into one another. this would mean that if the objects were move very slowly and the objects had time to slide back together the coefficient would increase. In this case the coefficient of friction would be directly dependent on velocity. This is not the case, it is simply a question of whether the objects are moving or not.

[IsaacC]

Given the distinction, the coefficient of friction doesn't seem to describe the "roughness" of a surface. Rather, it seems to numerically describe the magnitude of the "shifting" between two surfaces which makes sense considering that the coefficient of friction is a ratio ( Ff = u*Fn; Ff/Fn = u) and has no base units.

[Sarjeelbhat]

Please upload a topic on chemical effects of electric current

[timothyjohnson]

This is why is so hard to get up and moving off the couch when you live a sedentary lifestyle. I came here because I have problems with my own inertia

[romnicklor]

Can length contraction be a valid intuition? Because moving objects are shorter than their proper length, there would be less contact between the surfaces.

[nd]

Acceleration depends on force and mass right? Or does it depend on velocity too? Is the force required to accelerate the same block if it is moving or stationary different?

[chephybio]

Well this intuition confuses me
As there are a lot of questions against this intution :
1. If this intuition is correct then it should mean that if i take a block and make that block slide over a surface multiple times then the kinetic friction should decrease
As the surface becomes more even.
2. If this intuition is correct then, if i take multiple block of same material and mass and slide it on the the same surface then the kinetic friction coefficient is going to be different as the unevenness is going to be different for the blocks even though they are of same material.
But its not true
There are further arguments that can be placed here
But later i think i can answer all my questions to some extent:
We should consider that the friction depends more one the vanderwall force of attraction as compared to the unevenness of the body
So unevenness depends but not much .
Correct me if i am wrong

[Cyno]

khan are you going to be doing centripetal force?

[mayko]

I think that static friction is like a head start that's why it's bigger in value. Also the box with constant velocity has already overcome a bit of the friction.

[mtdeezy]

This reminds me of the experiment that the Mythbusters did that tested the myth that driving faster on a dirt road makes for a smoother ride than driving slowly.

[Phagocytosis]

Well, there's various types of bonds, but it sounded most like Van der Waals interactions or something.