why coefficient of kinetic friction is smaller than coefficient of static friction?

[just86]

by the equations:

fS = μsR

fK = μKR

 

why μK < μs ?

[thedarkshade]

by the equations:

fS = μsR

fK = μKR

 

why μK < μs ?

I think it's the other way around actually! The coefficient of kinetic friction is BIGGER than static friction. Because with the increase on friction coefficient the body starts moving. The bigger the coefficient the faster movement!

[just86]

The bigger the coefficient the faster movement?

The bigger the coefficient the larger the friction if it is under the same normal force ®

[insane_alien]

no he is right, this is usually the case because when an object is stationary the surfacesridges and bumps intermesh slightly(since they are not perfectly smooth) when it is moving they cannot do this so the force they can apply is smaller. of course, one the velocity is up high enough this will not matter and you will get a higher force of friction anyway.

[thedarkshade]

Let me make this simplier!

 

At school we did a very simple experiment! We used different materials and placed them at different angles! The higher the angle, the bigger the friction coefficient, and all the materials started falling down! And all the results were generally the same! With the increase on the friction coefficient each material started to move! That's why kinetic friction coefficient is bigger than static one!

 

Of course different materials had different numerical values but they all started moving with high friction coefficient! You can try that experiment too if you want, I'll give you the formula, just let me know!

[just86]

if the experiments are carried out on a flat surface

 

the object starts to move if the applied force is greater than the limiting static friction.

 

once the applied force exceeds the limiting static friction, the object starts to move, kinetic friction appears.

 

in this case, is it valid that kinetic friction < limiting static friction(or static friction)

 

this experiment is done under same normal reaction force.

[swansont]

Let me make this simplier!

 

At school we did a very simple experiment! We used different materials and placed them at different angles! The higher the angle, the bigger the friction coefficient, and all the materials started falling down! And all the results were generally the same! With the increase on the friction coefficient each material started to move! That's why kinetic friction coefficient is bigger than static one!

 

Of course different materials had different numerical values but they all started moving with high friction coefficient! You can try that experiment too if you want, I'll give you the formula, just let me know!

 

You are increasing the force, not the coefficient.

 

 

Friction is actially quite complicated; it has to do with the forces of surface irregularities and adhesive forces of molecular bonds. But there's a decent conceptual explanation of the difference here, in the last several paragraphs.

[just86]

You are increasing the force, not the coefficient.

 

 

Friction is actially quite complicated; it has to do with the forces of surface irregularities and adhesive forces of molecular bonds. But there's a decent conceptual explanation of the difference here, in the last several paragraphs.

 

i think i know what u are trying to say.

 

thanks !

 

thanks all of you!!

[thedarkshade]

once the applied force exceeds the limiting static friction, the object starts to move, kinetic friction appears.

 

in this case, is it valid that kinetic friction < limiting static friction(or static friction)

It's kinda creepy what you're saying! Friction after all is a fore that acts against the movement! This is why objects won't move infinitely when a force acts on them only once! And if you take a more logical approach to this, you'll see that what you're saying actually makes no sense! Think about Newton's third law. The action force is equal to reaction force. So with the increase in the force that makes the object move, the fiction force must increase too so the coefficient increases!

[insane_alien]

Friction after all is a fore that acts against the movement!

 

not always, stack two boxes on top of each other. push the bottom one. there is friction between the top and bottom boxes but it acts in the direction of motion.

[Mr Skeptic]

not always, stack two boxes on top of each other. push the bottom one. there is friction between the top and bottom boxes but it acts in the direction of motion.

 

Newton's Second Law. There is always friction in both directions. The friction on the moving object is against the direction of movement, and the friction on the stationary object is with the direction of the moving object's movement. Putting the friction on top or on the bottom makes no difference.

[swansont]

It's kinda creepy what you're saying! Friction after all is a fore that acts against the movement! This is why objects won't move infinitely when a force acts on them only once! And if you take a more logical approach to this, you'll see that what you're saying actually makes no sense! Think about Newton's third law. The action force is equal to reaction force. So with the increase in the force that makes the object move, the fiction force must increase too so the coefficient increases!

 

 

The coefficient refers to the maximum frictional force that will be present. The force can change, but the coefficient does not.

[thedarkshade]

The coefficient refers to the maximum frictional force that will be present. The force can change, but the coefficient does not.

 

Well that's different from what we learned at school. Let's refer to this equation that I learned at school and that is for friction force:

 

F=Nµ (N=Q=mg) /:N

 

µ=F/N

 

So by this second equation we can clearly see that with the increase in friction force, we also increase the friction coefficient!

[Cap'n Refsmmat]

F in that equation represents the maximum friction force, not the force at any given point in time.

[swansont]

F in that equation represents the maximum friction force, not the force at any given point in time.

 

Precisely.

 

[math] \mu_{static} = \frac{F}{N}[/math] holds for the point where motion starts

 

[math] \mu_{kinetic} = \frac{F}{N}[/math] holds for motion

[tvp45]

And, there is an exception. The coefficient of kinetic friction for aluminum on dry aluminum is greater than the coefficient of static friction. (Avallone and Baumeister)

[bsn117]

Well that's different from what we learned at school. Let's refer to this equation that I learned at school and that is for friction force:

 

F=Nµ (N=Q=mg) /:N

 

µ=F/N

 

So by this second equation we can clearly see that with the increase in friction force, we also increase the friction coefficient!

 

im sorry, but everything you have said in this thread is wrong. static friction is greater than kinetic friction. the coefficient of friction is a ration of the friction force to the normal force. thats why when you did your experiment with the incline plane and the object, once the object gathered enough force to overcome the static friction, it continues to accelerate, because the kinetic friction is smaller than the static friction, causing the net force to be > 0. its actually quite simple. i'm in 10th grade, and we just learned this. if you think about it logically, it makes no sense for kinetic friction to be greater than friction. if it is, then it will prevent the object form moving in the first place, and that makes it static friction. im not trying to put you down, make fun of you, or make you mad. im just saying that whatever you "learned" about it in school is wrong. i suggest you go back and study.g

[TonyMcC]

When I was at school we called static friction stiction with the idea that to some extent two stationary surfaces will stick together. Depending on smoothness this could be due to humps and hollows (rough) right down to atomic bonding (very smooth). Stiction was found experimentally to be greater than friction between two surfaces in sliding contact. Out of interest I have just googled "stiction" and apparently the term is still used.

[lemur]

no he is right, this is usually the case because when an object is stationary the surfacesridges and bumps intermesh slightly(since they are not perfectly smooth) when it is moving they cannot do this so the force they can apply is smaller. of course, one the velocity is up high enough this will not matter and you will get a higher force of friction anyway.

I think this is the best answer on this thread. Another example could be a chair with rubber feet. When you start pushing the chair, the initial force goes into compressing and stretching the rubber until enough force is reached to cause the rubber to slide over the surface. Once it begins to slide, it doesn't have the opportunity to anchor as strongly because the moment it starts to stick, its momentum drags it back into motion, overcoming the friction. If it slows down enough to "catch" and anchor, it requires that much more energy to overcome the sticking.

 

I think the relative ease of gliding as an object's speed increases also has to do with air and loose particles forming a thin fluid layer that acts as lubricant. If you place a box on top of a number of perfectly round ball bearings, it glides more smoothly than if the ball bearings are hexagonal. So you're dealing with the force needed to lever oddly shaped particles of sand/dust until they start to roll at which point they bounce around between the bottom of the moving box and the surface it's gliding over.