/sci/ - Science & Math

>>11367481
Because you need to apply as much if it already momentus. Static friction force demands you to both remove the object from its fit in the "surface's micro-cracks" as well as to give it speed; kinetic only requires you to keep the speed up despite very briefly 'fitting the microcracks' — since there was already some speed you put there initially, the object's 'microcracks' don't fit as much into the microcracks of the surface as it fit when it was stopped, so it requires less force to get it out of there and keep it going.
Black is the object and red is the surface.

bump

It's pretty simple. There is static friction and kinetic friction because an object can be static relative to the surface it sits on or it can be in motion.
The static frictional force can be anything between 0 and μN depending on the external force applied. Once the applied force that opposes friction surpasses μΝ, the object is in motion and now experience kinetic friction. Notice that we can never have the coefficient of kinetic friction be greater than coef. of stat. fric. because then getting an object to move would be impossible. There is no theoretical reason that the two coefficients couldn't be equal, however, though we typically observe it that μ_k<μ_s.
>okay but why
Not science. There isn't more to it. Static and kinetic friction are just macroscopic approximations that account for a shitload of microscopic interactions at the interface between the materials as shown in the pic here. >>11367525
What more could you be confused about?

>>11367525
>>11368012
/thread

>>11368012
Wouldn't static friction simply be trivial if [math] \mu_k > \mu_s [/math]? It would still move if the applied force exceeded [math] \mu_k N [/math].

>>11368018
https://physics.stackexchange.com/questions/541/can-the-coefficient-of-static-friction-be-less-than-that-of-kinetic-friction

>>11368021
Okay, so the top posts basically agree with my assessment. Static friction force being less than kinetic force doesn't make any sense. If it were less, it really would be equal by the definition of static friction force.

>>11368023
Thats what I said in my first post, butthole

>>11368012
>Not science. There isn't more to it.
You are mistaken.

momentum, inertia, etc

>>11367525
>the object's 'microcracks' don't fit as much into the microcracks of the surface as it fit when it was stopped, so it requires less force to get it out of there and keep it going
This is a common explanation, but it is not at all obvious that it is correct.
>>11368012
>Static and kinetic friction are just macroscopic approximations that account for a shitload of microscopic interactions at the interface between the materials. What more could you be confused about?
Is crack-settling the mechanism of friction, or is it Van-Der-Waals sticking? Is the time scale for crack settling really large compared to the sticking and unsticking time for contacts? You want a quantitative model that tells you what happens at the surface.
>Not science. There isn't more to it.
What the fuck? What the fuck are you doing? You do science? Your science is whack.

>>11368657
>Is crack-settling the mechanism of friction, or is it Van-Der-Waals sticking?
All of the above.
>Is the time scale for crack settling really large compared to the sticking and unsticking time for contacts?
The ratio of the length scale of grain boundaries and microscopic cracks to whatever is happening on the atomic level is ~10^3-10^4, so I would crudely estimate that yes, the time scale for crack settling is large compared to intermolecular forces.

bump!

>>11369432
All of your questions have been thoroughly answered.

>>11368657
What's the reasoning for these queries?

>>11369019
>all of the above
>the ratio of the length scale of grain boundaries and microscopic cracks to whatever is happening on the atomic level is ~10^3-10^4, so I would crudely estimate that yes, the time scale for crack settling is large compared to intermolecular forces.
>>11369446
>thoroughly

>>11369019
I wanted to fuck that fox when I was like 5 years old.

>>11368012
>Not science. There isn't more to it.
t. Engineer

>>11371526
>not wanting to fuck jenny the robot
Faggot.

>>11367481
1/2 it's either moving or it isn't

>>11371526
>

>>11367525
I went full sperg in the first phrase, I meant 'Because you don't need to apply as much force if it already has momentum'