# Why is it so hard to explain friction?

• I
While in class, my Thermodynamics professor came up with an interesting fact, that neither I nor my colleagues were ever aware of: While you may have heard that friction influenciates the movement in Newtonian Mechanics, no one really knows what friction is.
And so I have been wandering why it is so hard to explain something that is so natural to us? What are the physics behind this? Hope you can help clarify my doubt.

Drakkith
Staff Emeritus
Err, what? As far as I am aware, we know exactly what friction is. Can you elaborate a bit on what you mean?

anorlunda
Staff Emeritus
Friction is an average behavior. At the molecular level, surfaces are very irregular. Stressing a crystal lattice by pushing it, is a molecular level problem.

I think what the professor meant, is that without complete knowledge of the atoms in the materials, we can not have detailed knowledge of the forces.

In particle physics, quantum mechanics and Newton's laws apply and there is no friction.

But on a macro level, the average behavior is very well described by friction.

russ_watters
Mentor
Yeah, just like people say no one really knows what gravity is.

Khashishi
Staff Emeritus
Another "I don't understand it so nobody understands it" thread.

sophiecentaur
A.T.
I think what the professor meant, is that without complete knowledge of the atoms in the materials, we can not have detailed knowledge of the forces.
Or that we don't have a useful general predictive model for friction coefficients. We usually derive them empirically, without knowing/caring what actually happens on the microscopic level.

Staff Emeritus
Or that we don't have a useful general predictive model for friction coefficients.

The key word is "useful". There certainly is a working microscopic model. It's complicated because what we call "friction" corresponds to multiple microscopic processes. It's complicated because even in simple systems many different materials are involved: a piece of aluminum can have aluminum, three different oxides, and many different contaminants, all contributing. It's complicated because you have grain boundaries, orientations, and, and, and...

But this is saying that this is a complex problem, not that there is a fundamental lack of understanding. Nobody would say we don't understand atomic physics just because we can't calculate the color of lead (82 electrons) from the Schroedinger Equation.

Aaron Crowl, Almada R, russ_watters and 1 other person
sophiecentaur