Why is (Kinetic friction)= fk≤fs (Static friction) ?

[andyrk]

Why is (Kinetic friction)= f_k≤f_s (Static friction) ? I understood the "equal to" case but how is the "less than" case possible?
Similarly is this related to why is μ_k≤ μ_s ?

 

[tolove]

Good question! Try here:

https://www.physicsforums.com/showthread.php?t=140426

 

[haruspex]

Good question! Try here:

https://www.physicsforums.com/showthread.php?t=140426

I looked at that, but unfortunately there's about as much misinformation as worthwhile comments there. And I couldn't find any that match what I've always taken to be the explanation: that there's an energy threshold to be overcome.
Imagine e.g. that the two surfaces are made of materials which, if completely flat, would have no friction at all. But instead of being flat, each consists of parallel ripples. At rest, the two sets of ripples interlock. A small amount of energy needs to be invested in lifting the block over the ripples, but that energy is then available to get it over the next ripple, and so on.
It needn't be physical ripples - it could be some electrostatic bonding, but the concept is the same.
So I did a quick search and found this http://www.khanacademy.org/science/...on-on-static-and-kinetic-friction-comparisons

 

[tolove]

I looked at that, but unfortunately there's about as much misinformation as worthwhile comments there. And I couldn't find any that match what I've always taken to be the explanation: that there's an energy threshold to be overcome.
Imagine e.g. that the two surfaces are made of materials which, if completely flat, would have no friction at all. But instead of being flat, each consists of parallel ripples. At rest, the two sets of ripples interlock. A small amount of energy needs to be invested in lifting the block over the ripples, but that energy is then available to get it over the next ripple, and so on.
It needn't be physical ripples - it could be some electrostatic bonding, but the concept is the same.
So I did a quick search and found this http://www.khanacademy.org/science/...on-on-static-and-kinetic-friction-comparisons

Very interesting video. Do you have an idea of how accurate his presentation is? He ends it by saying that it's still a subject open to discussion.

Thank you for the link!

 

[haruspex]

Do you have an idea of how accurate his presentation is?

Only that it jibes with what I thought up myself.

 

[andyrk]

Thanks a lot for the link!