Can kinetic friction be greater than static friction? How can this be tested?

[SelmerSaxMan]

Hi all. My physics teacher told me a while ago, that, under certain conditions a situation could be made in which kinetic friction is greater than static friction. He didn't know how this was done. Would anyone here be able to tell me how one would set this situation up and how he would measure the results?

 

[SelmerSaxMan]

By the way, this isn't homework related, this is for my own interests, if that is which makes a difference in inspiring a reply to the question begged.

 

[TiBaal89]

Since our understanding of friction is still very incomplete its hard for me to say why... but I don't think that's possible. I'm interested to see what someone else might chime in with.

 

[benzun_1999]

i think static friction can be equal to kinetic friction but not equal.

 

[benzun_1999]

sorry
kinetic friction can be equal to static friction but not greater than static friction.

please forgive me, mistyping.

-benzun

 

[kant]

we know that the proventive force of a abject in motion due to friction is a function of the normal force. What if you have a situcation where is there is a varying( uneven) level of such normal force? This wil will cause an varying( uneven) level of static v.s kinetic frictional forces? Can this work?

 

[zeta101]

hmmm interesting question. I would say it would have something to do with a special material for the object that's moving, such that, as the object moves and the contact surface heats up due to friction, the surface undergoes some physical or chemical change so that its coefficient of friction increases.

This would mean if you started to push the object on a flat plane so that it just starts moving (and then keep pushing with that same constant force), then as the heat builds up and the critical temperature is reached that the object halts to a stop even though you are still applying the same constant force...

(kant is also correct in a way, for example if you increase the mass of the object that is moving (say a bucket of sand that you continally add sand to)).

 

[maverick6664]

It's impossible.

Imagine if kinetic friction is greater than static friction. If you pull (or push) a box of a certain weight on a flat floor, gradually increasing pulling force. When the force becomes greater than the static friction force, the box will begin to move, but at that point due to greater kinetic friction, this box must not be moving. This is a contradiction and the reason static friction must be always greater than or equal to kinetic friction.

Hope this helps.

I've read this discussion in a Feynman's book (Feynman's lectures on physics). I think it's in vol. 1. But alas, it's not at hand now.