How does kinetic friction occur between two moving bodies?

[parshyaa]

Could anybody explain what happens at the molecular level when one of the two bodies in a contact starts moving w.r.t other, i mean in this process what causes kinetics friction force, i tried myself to think of it but i could not come up with any, so please help me.

 

[BvU]

Consider a bigger-than molecular level: irregularities in the surface work like hooks and have to be pushed up or broken to keep going.
Story goes a long way when zooming in. Changes for 'perfectly' smooth surfaces: but even then there is some viscous medium in between.

 

[anorlunda]

Here's a picture to illustrate @BvU 's answer metaphorically. Imagine two rasps with rough sides facing each other sliding past each other. Think of the resistance compared to sliding two smooth things.

 

[parshyaa]

Wow i got it
Thanks @BvU and @anorlunda
We could also imagine it by keeping one hand stiff and finger stretched out and then moving another hand's finger over the fingers of that hand.

So another question raised in my mind,
Do kinetic friction always acts in opposite direction of motion?(i am only speaking for kinetic friction)

 

[BvU]

In practice: yes.

A bit semantical: if it doesn't, we don't call it friction

The springs in the microscopic model in the link push a much smaller part of the time than pull.

 

[Jay Addy]

So another question raised in my mind,
Do kinetic friction always acts in opposite direction of motion?(i am only speaking for kinetic friction)

Friction always acts to oppose any relative motion between surfaces.

 

[parshyaa]

In practice: yes.

A bit semantical: if it doesn't, we don't call it friction

The springs in the microscopic model in the link push a much smaller part of the time than pull.

I think only kinetic friction is in opposite to the direction of motion, otherwise for example consider two block A and B, A is on B and B is pulled by a force F so both block accelarates with accelaration 'a' in the direction of force but we noticed that there is no force acting on upper block but then also it is accelarating, therefore in this case friction is acting to accelarate block A, here friction is not in opposite to the direction of motion.
Here friction is static friction

 

[anorlunda]

I think only kinetic friction is in opposite to the direction of motion, otherwise for example consider two block A and B, A is on B and B is pulled by a force F so both block accelarates with accelaration 'a' in the direction of force but we noticed that there is no force acting on upper block but then also it is accelarating, therefore in this case friction is acting to accelarate block A, here friction is not in opposite to the direction of motion.

OK, restate it as friction always opposes relative motion. Since all motion is relative, in your example above there is a frame of reference in which one of the object is not moving. But nonzero relative motion between objects is nonzero in all frames.

 

[Jay Addy]

I think only kinetic friction is in opposite to the direction of motion, otherwise for example consider two block A and B, A is on B and B is pulled by a force F so both block accelarates with accelaration 'a' in the direction of force but we noticed that there is no force acting on upper block but then also it is accelarating, therefore in this case friction is acting to accelarate block A, here friction is not in opposite to the direction of motion.
Here friction is static friction

The friction itself is always opposed to relative motion. The force that pulls on the object is creating the acceleration. The friction is working against the relative motion, which you achieve by applying a said force "x" to block B. The reason why block A might move in this scenario, is because of the fact that block B's weight (G=m*g) sits on top of block A, "forging" the two blocks together. (This is obviously an exaggeration). When block B accelerates, block A accelerates with it. Think of it this way: If you have a book on a table with wheels, and you push the table forward. What happens to the book? It moves with the table. If the friction between the book and the table wouldn't oppose relative motion, then the book would slide off the table.

 

[A.T.]

The friction is not accelerating block A.

Of course it is.

 

[parshyaa]

Think of it this way: If you have a book on a table with wheels, and you push the table forward. What happens to the book? It moves with the table. If the friction between the book and the table wouldn't oppose relative motion, then the book would slide off the table.

So what is making book to accelarate with table as in same direction as table is moving

 

[Jay Addy]

So what is making book to accelarate with table as in same direction as table is moving

I misunderstood your question and got it all wrong, I'm sorry. Both the drag and the friction between the two blocks will be responsible for moving the bottom block (if a force is applied to the top) or vice versa. On the example above with the book and table, the drag and friction between the book and table is moving the book relative to the table. If you would like to read more about "Two Body problems," then you'll be able to find some good articles on google. They can be broken down into free body diagrams (individual diagrams) if you would like to focus on the forces that act on the individual objects, which might be better for the questions you've asked.

As A.T. mentioned, what I said wasn't correct, and I stand corrected. Have a great evening.

Jay.

 

[parshyaa]

No budy answered my final question yet
Do kinetic friction always acts in opposite direction of motion
I think yes, i need more opinions

 

[jbriggs444]

No budy answered my final question yet
Do kinetic friction always acts in opposite direction of motion

The answer has been given several times. Kinetic friction acts opposite to relative motion between surfaces in contact.

The only exception that comes to mind would be surfaces with direction-specific features. Something like a pair of single cut (parallel diagonal grooves) metal files pulled against each other.

 

[parshyaa]

Could any budy tell me the intuition behind Why friction is proportional to normal force?

 

[jbriggs444]

Could any budy tell me the intuition behind Why friction is proportional to normal force?

In my opinion, the cause is irrelevant. It is an engineering approximation (the "Coulomb model"). It is not a fundamental principle of physics and is not 100% accurate.

 

[parshyaa]

In my opinion, the cause is irrelevant. It is an engineering approximation (the "Coulomb model"). It is not a fundamental principle of physics and is not 100% accurate.

https://physics.stackexchange.com/q...ce-dependent-on-normal-reaction/260997#260997
I liked john rennie's answer, what do you think of that?

 

[jbriggs444]

https://physics.stackexchange.com/q...ce-dependent-on-normal-reaction/260997#260997
I liked john rennie's answer, what do you think of that?

It seems plausible enough and is exactly the sort of theory that I would come up with. Which is one reason why I do not trust it. It might be right. It might be wrong. It might be a partial explanation of something far more complex.

Fortunately, the question of its correctness does not normally arise. For the purposes of solving first year kinematics problems with boxes sliding on other boxes, inclined planes and truck beds it is enough to have a coefficient of static or kinetic friction and use it.

 

[parshyaa]

It seems plausible enough and is exactly the sort of theory that I would come up with. Which is one reason why I do not trust it. It might be right. It might be wrong. It might be a partial explanation of something far more complex.

Fortunately, the question of its correctness does not normally arise. For the purposes of solving first year kinematics problems with boxes sliding on other boxes, inclined planes and truck beds it is enough to have a coefficient of static or kinetic friction and use it.

Yes you are right, for solving problem this is not required, but i don't know why but questioning concept has become my hobby, even if i don't want to raise a question a question immediately bumps in my head related to the theory part, i want to become theorotical physics and to do so i have to get into a good university and without practicing questions this could not be possible, i could solve questions easily but i don't know why do i stuck to a particular concept for a long time.

 

[anorlunda]

https://physics.stackexchange.com/q...ce-dependent-on-normal-reaction/260997#260997
I liked john rennie's answer, what do you think of that?

Curiosity and research are virtues and are encouraged. But, you missed the following that Rennie said about his own post.

It's important to understand that this is all a rather handwaving approach so it is only an approximation.

hand waving

noun
1.
insubstantial words, arguments, gestures, orations used in an attempt to explain or persuade.

That is why you may encounter frowns from physicists. Handwaving arguments are not welcome among physicists, even though this one seemed to help you understand.

There is no physics law that describes friction. Friction is merely an useful approximation that ignores the real molecular physics. Therefore, you might have found a friendlier response in the engineering forums than in the physics forum.

Engineers focus on what is useful. Friction is useful in engineering so we engineers make use of with without regard to the physics. The same is true with Ohm's law. It is extremely useful but nearly impossible to derive from quantum physics. We are happy to use it as an approximation, without regard to the physics.

 

[parshyaa]

Curiosity and research are virtues and are encouraged. But, you missed the following that Rennie said about his own post.That is why you may encounter frowns from physicists. Handwaving arguments are not welcome among physicists, even though this one seemed to help you understand.

There is no physics law that describes friction. Friction is merely an useful approximation that ignores the real molecular physics. Therefore, you might have found a friendlier response in the engineering forums than in the physics forum.

Engineers focus on what is useful. Friction is useful in engineering so we engineers make use of with without regard to the physics. The same is true with Ohm's law. It is extremely useful but nearly impossible to derive from quantum physics. We are happy to use it as an approximation, without regard to the physics.

Yes you are right, this is why i said i liked his answer, I didn't said that i agree with him.i posted his answer here to get opinion on his approach,

 

[parshyaa]

@jbriggs444
I have another doubt
Is friction always independent of surface area? Its hard to move solid cube then ball of same mass. I think friction depends on real contact area between the surface asperities then the geometric contact between the surface.

 

[Jay Addy]

@jbriggs444
I have another doubt
Is friction always independent of surface area? Its hard to move solid cube then ball of same mass. I think friction depends on real contact area between the surface asperities then the geometric contact between the surface.

I think what you're trying to say is: Is friction always dependent of surface area?

To answer that question: No. Not that I am aware of.
A larger contact area between two objects will create a larger source for the frictional force, however, given a larger surface area, the pressure between the two surfaces held together by a given force will drop. The increase in friction generating area and the offset of pressure is most likely equivalent. The friction that is left will depend on the frictional coefficient of the objects and the force holding them together. If you were to increase the force, while also increasing the surface area to keep constant pressure, then the frictional force between the objects would increase.

"Its hard to move solid cube then ball of same mass. I think friction depends on real contact area between the surface asperities then the geometric contact between the surface."

I suggest you read Gert's answer to a similar question:

All things being equal and in the presence of sufficient friction, the velocity (translation) of a sphere and a cube are the same. Both cases require the same amount of work but in the case of a (non-tumbling) cube part of that work is lost as friction work and thus not conserved.

Source: https://physics.stackexchange.com/questions/219313/sliding-and-rolling-friction

 

[jbriggs444]

Is friction always independent of surface area?

No. It is only approximately true.

 

[Nidum]

And if there is any lubricant present between the surfaces involved then it is not even approximately true .

Doesn't take much lubricant to make the so called Laws of Friction totally unreliable - a moisture film is sometimes enough .

 

[parshyaa]

No. It is only approximately true.

So if surface area is really big then friction will depend on surface area, i think that its approximately independent of surface area because when surface area increases then real area of contact at microscopic level also increases and as real area of contact increases then pressure on asperities decreases and the deformation of asperities takes place slowly and thus real area of contact increases with slow rate and thus friction increases with slow rate then compare to the friction between smaller area of contact between the surface.
And if area of contact is huge then it will increase area of contact with big amount and friction will increase with high rate
This is just a theorotical approach i understood till now, I don't have proof for this, but till now this satisfies me

 

[Nidum]

The story goes that a large surface is just an assembly of small surfaces all with the same friction characteristics and the normal load on the large surface just gets shared out as small loads on the small surfaces .

 

[jbriggs444]

I think this is a wrong story
Because real area of contact between two surface is much lesser than the geometrical area of contact between the surface

Your argument here agrees with what @Nidum is saying. What is your disagreement?

 

[parshyaa]

Your argument here agrees with what @Nidum is saying. What is your disagreement?

@Nidum , sorry i misunderstood your point

 

[parshyaa]

How would you solve this question:
A body slipping on a rough horizontal plane moves with a deceleration of 4m/s^2. What is the coefficient of kinetic friction between the block and plane ?
Please note that this is not a HW question
I am asking this, because i think that in this case you will take -4 = -f(static friction)/m
And thus 4= f(static friction)/m
f(static friction) = μN = μmg
Therefore μ=4/g
In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating
Then how will you explain it at molecular level.

 

[anorlunda]

A body slipping on a rough horizontal plane moves with a deceleration of 4m/s^2.

In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

You said opposite things. First you say it is decelerating (slowing down; eventually to stop). Then you say it accelerating (speeding up). You are confusing yourself.

 

[CWatters]

In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

Kinetic friction resists motion, that is correct.
The velocity is positive so F_{kinetic friction} is negative and so the acceleration is negative (=deceleration).

PS: F_{kinetic friction} is not a constant force. As soon as the velocity reaches zero F_{kinetic friction} becomes zero and is replaced by static friction. So F_{kinetic friction} cannot make the block accelerate in the opposite direction.

 

[parshyaa]

If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case

 

[jbriggs444]

View attachment 215847
If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case

The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?

 

[parshyaa]

The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?

Yes you are right, my confusion has gone now

I could solve problem containing two or three surfaces but solving questions containing only two surfaces sometimes makes me confuse but now i am cleared
Thanks @jbriggs444 and @anorlunda