Where does static friction go?

[guitarfrk451]

I understand to overcome static friction..you must have an unbalance force more than the friction itself after you overcome static it changes to kinetic. Where does the static friction go? I understand it changes but can someone help me try to understand this a little bit better?

[Kyrisch]

 

Tiny imperfections in the surface mesh like gear teeth. Once these "teeth" are uncoupled, the surfaces begin sliding and it is an entirely different process. This is why the kinetic and static coefficients of friction are so different.

[guitarfrk451]

yeah i get that part...where does it go though? That is after it has overcome the static?

[Cap'n Refsmmat]

It doesn't "go" anywhere. It's just that it no longer acts. Those little imperfections aren't meshing -- they're bouncing against each other now that it's moving.

 

You could say it's the same thing happening (imperfections grinding against each other), just that when the surfaces start moving they bounce, making the amount of friction go down. Hence the change from one coefficient to another.

[guitarfrk451]

and kinetic's always less than static..correct?

[Cap'n Refsmmat]

Pretty much 99.9% of the time. There might be a situation where kinetic is greater, but I can't think of any.

[guitarfrk451]

I'm not sure..It's just hard for me to grasp this concept. I know about it, but i just can't "see" it in my head. I like it better when I can fully understand it. But unfortunately that's rarely the case in physics..

[Cap'n Refsmmat]

Think of it this way:

 

Suppose I have a table sitting on the floor. I push on the table. The table's static friction resists because the small unevennesses in the table and the floor grip each other, keeping the table from moving. That's static friction.

 

Then I push harder, and I overcome the static friction. I pushed hard enough that the bumps started sliding past each other. (It's a lot like two gears: the teeth normally intertwine, making the gears rotate together. If you twist hard enough, the teeth start coming apart.) Once they're sliding, they don't have a chance to "grip" each other as much. (Once the teeth have come apart and the two gears are slipping, their motion will keep the teeth from properly getting back together and gripping each other.) Hence the friction is less.

[guitarfrk451]

Oh i see..thanks

 

-Matt