# I How kinetic friction works

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1. Nov 24, 2017

### 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.

2. Nov 24, 2017

### 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.

3. Nov 24, 2017

### Staff: Mentor

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.

4. Nov 24, 2017

### 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)

5. Nov 24, 2017

### 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.

6. Nov 24, 2017

Friction always acts to oppose any relative motion between surfaces.

7. Nov 24, 2017

### parshyaa

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

8. Nov 24, 2017

### Staff: Mentor

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.

9. Nov 24, 2017

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.

Last edited: Nov 24, 2017
10. Nov 24, 2017

### A.T.

Of course it is.

11. Nov 24, 2017

### parshyaa

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

12. Nov 24, 2017

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.

13. Nov 24, 2017

### 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

14. Nov 25, 2017

### jbriggs444

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.

Last edited: Nov 25, 2017
15. Nov 25, 2017

### parshyaa

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

16. Nov 25, 2017

### jbriggs444

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.

17. Nov 25, 2017

### parshyaa

18. Nov 25, 2017

### jbriggs444

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.

19. Nov 25, 2017

### parshyaa

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.

20. Nov 25, 2017

### Staff: Mentor

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.