Usually when we deal with physical problems, don't we say that the friction on a smooth surface is zero? Then how does the book say that the smoother the surface is, the greater the friction coefficient is So according to f = um, doesn't it mean that the smoother the surface is, the greater the friction is

Usually when we deal with physical problems, don't we say that the friction on a smooth surface is zero? Then how does the book say that the smoother the surface is, the greater the friction coefficient is So according to f = um, doesn't it mean that the smoother the surface is, the greater the friction is

The friction coefficient is relative to a pair of friction pairs. There are two main explanations for the nature of friction, namely, concave convex engagement theory and friction adhesion theory
From the name, we can roughly imagine its content. Through experiments and calculations, it is found that the above two theories can produce friction, and the mechanism proposed by the adhesion theory is more common than that proposed by the meshing theory. But in different materials, the performance of the two mechanisms is biased: for metal materials, the friction is mainly caused by adhesion; for wood, the friction is mainly caused by meshing
The theory of concave convex engagement can explain the phenomenon of small friction on smooth surface. The phenomenon of large friction coefficient on smooth surface can also be explained by adhesion theory. The reason for this contradiction is that different friction pairs adopt different friction forming mechanisms
In fact, they only contact each other where the surface is convex, but most of them are not. The actual contact micro area is far less than the apparent macro area. The friction resistance is proportional to the actual contact area (not to the apparent macro area), The actual contact area is directly proportional to the positive pressure on the surface, so the friction force is directly proportional to the positive pressure. This is the reason why the friction force formula F = μ n in the middle school physics textbook defines the scale factor μ as the dynamic friction coefficient between two materials
To smooth the surface is another way to increase the actual contact area and the friction force, which is reflected in the increase of the friction coefficient in the friction formula
I'm a little disappointed. In fact, the nature of friction is still under discussion

The physical control variable method is used to prove which factors are related to the pressure
When doing this problem, we don't know which factors are related to the pressure, so how to know which physical quantities should be controlled?

First guess with which a few quantity, according to the usual experience, feeling and so on
Then control one or more of them to remain unchanged, leaving only two variable quantities (one is the pressure, the other is your guess)
And then we know what the relationship between the pressure and the quantity is
By analogy, the relationship between pressure and other quantities is obtained

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If you can't go to the birthday party because of something, thank you and explain the reason.

Dear friend,Thank you for your asking.I would love to,but I am really a little bit busy that day.I have to do something important.I am very sorry that I can not go to the there.I'm really sorry about ...

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