# I How kinetic friction works

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1. Nov 25, 2017

### parshyaa

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,

2. Nov 27, 2017

### 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.

3. Nov 28, 2017

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:

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

Last edited: Nov 28, 2017
4. Nov 28, 2017

### jbriggs444

No. It is only approximately true.

5. Nov 28, 2017

### 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 .

6. Nov 28, 2017

### parshyaa

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

7. Nov 28, 2017

### 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 .

8. Nov 28, 2017

### jbriggs444

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

9. Nov 28, 2017

### parshyaa

@Nidum , sorry i misunderstood your point

10. Nov 30, 2017

### 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.

11. Nov 30, 2017

### Staff: Mentor

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.

12. Nov 30, 2017

### CWatters

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

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

13. Nov 30, 2017

### 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

14. Nov 30, 2017

### jbriggs444

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

15. Nov 30, 2017

### parshyaa

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