In high school physics the phenomenon of friction is reduced to the classical
friction laws of Leonardo da
Vinci, Guillaume Amontons, Leonard Euler und Charles Coulomb. Leonardo da Vinci made experiments
on an inclined plane. He found that friction is independent on the area of contact. Amontons did experiments on a horizontal surface and measured the friction force
with a spring. He found that friction is proportional to the normal force and independent on the
area of contact. He called the proportional factor friction constant. While Leonardo tested static friction Amontons
dealt with kinetic friction. It was found by Leonhard Euler that one has to distinguish between static
and kinetic friction, because it is not possible to cause a slow motion by slowly increasing the angle of an inclined plan.
Also Coulomb looked at the phenomenon of friction. He built an experiment, which allowed to measure kinetic
friction for different speeds. He found that friction is independent on the velocity.
The law of Leonardo appeard very paradox. Intuitively one would the friction force expect to be proportional to the area of contact.
The paradoxon was resolved by F. P. Bowden
and D. Tabor. They distinguished between the real area of contact
and the geometric visible area of contact. The real area of contact is only a fraction of the visible area
of contact. All experiments lead to the conclusion that friction is proportional to the real area of contact as intuitively expected.