# Friction

#### werehk

PHF Hall of Fame
I always thounght that friction is a very interesting thing as friction is independent of contact area and mass. We actually don't quite understand what friction is.

We are always taught that the surfaces are rough which gives rise to friction. However, this is contrary to the property that friction is independent of contact area right?

Could there be a better "model" that describes friction?

#### topsquark

Forum Staff
I always thounght that friction is a very interesting thing as friction is independent of contact area and mass. We actually don't quite understand what friction is.

We are always taught that the surfaces are rough which gives rise to friction. However, this is contrary to the property that friction is independent of contact area right?

Could there be a better "model" that describes friction?
In reality friction does depend on the contact area and the weight of the object. (In general it is not the weight that is a factor, but the normal component of the net force between the two surfaces.) However the simple model, which tends to be accurate in the average, is the standard $$\displaystyle \mu N$$ force. Why this is so I cannot say.

To be absolutely clear about this, the standard development of the friction forces is independent of the roughness of the two surfaces. (As I said $$\displaystyle \mu N$$ is the averaged version of what is really going on, and so we would expect it to also hold for two perfectly smooth surfaces as well.)

-Dan

• werehk

#### werehk

PHF Hall of Fame
Would there be anything related to the electrons of two surfaces contributing to the property?

#### SengNee

What I understood is actually there is no perfectly smooth surface. Even a smooth surface if we magnify it, we will see that it's also uneven. Friction is the produced at the high pressure point between 2 surfaces. I'm not sure about is it depends on electrons.

Rolling friction is static friction of kinetic friction?

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#### topsquark

Forum Staff
Would there be anything related to the electrons of two surfaces contributing to the property?
Speaking in terms of Quantum Physics, the "contact" between any two surfaces can be described in terms of the electron shells of the surface atoms interacting with each other. Unfortunately the equations for this situation are currently impossible to solve.

However there is one situation where you can see this directly. Take a piece of copper (any metal will do) and cut it into two pieces. Now, before any significant oxidation can start stick the two pieces together again. When you try to slide the two surfaces (or even pull them apart) you will find it to be surprisingly difficult. This is because the surface atoms in the two pieces "see" no reason that they belong to separate pieces and bond to each other. Obviously the bonding won't be perfect and you can in fact slide the two surfaces along each other (or pull them apart). But the initial force required is fairly high.

-Dan

• werehk and SengNee

#### topsquark

Forum Staff
Rolling friction is static friction of kinetic friction?
In a word, yes. -Dan

#### SengNee

In a word, yes. -Dan
That means rolling friiction is both kinetic and static friction?

One of my lecturer said that rolling friction is static friction as the point at the tyre suface is just steps on ground and then lifts up when successive point at the tyre surface steps on the ground, so that it is static.
But my another lecturer said that rolling friction is kinetic friction as the car is moving.
I'm very confuse and I can't find out the answer in the library.

#### DivideBy0

My teacher said rolling friction is mathematically static, I think it is because the surfaces of the wheel and the ground aren't actually rubbing together. This kinda makes sense because if a wheel rolls perfectly on a surface with static coefficient 0.5, then it will also roll with same speed on a surface with static coefficient greater than 0.5. As long as $$\displaystyle \mu N \geq F$$ then the wheel will roll and not slide.

#### SengNee

This kinda makes sense because if a wheel rolls perfectly on a surface with static coefficient 0.5, then it will also roll with same speed on a surface with static coefficient greater than 0.5. As long as $$\displaystyle \mu N \geq F$$ then the wheel will roll and not slide.
I don't understand here...

#### DivideBy0

I don't understand here...
Ok I think I bundled two arguments into one before:

1.
If you are dealing with static friction, as long as the frictional force is greater than the force being applied, then the object will remain at rest. If $$\displaystyle \mu_{static} N \geq F$$, the object will remain stationary.
It is the static friction which is making the wheels not slide, and this allows the wheels to roll.

2. (Evidence by experience)
If you are driving a car on a mostly smooth road and the wheels do not slide, then if you drive a car on a really rough road, the wheels also will not slide. If you experiment with this you see that the car moves at the same speed on both surfaces if you apply the same force to the back of the car. i.e. if the car is rolling, friction is independent of force applied.

This is not the same if you consider kinetic friction. With kinetic friction, the frictional force $$\displaystyle Fr$$ is proportional to the coefficient of friction $$\displaystyle \mu_{kinetic}$$ ($$\displaystyle F = \mu_{kinetic} N$$). This means that if a car wheel uses kinetic friction then it will roll significantly slower (or significantly more force will be required) on a rough road than a smooth road. This in practice is untrue, so a car wheel must use static friction.

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