Leonardo Da Vinci (1452-1519) stated the two basic
laws of friction 200 years before Newton even defined what force is.
Da Vinci simply stated that: 1) if the load of an object is doubled, its
friction with also be doubled; and 2) the areas in contact have no effect
on friction. These were two profound statements. The first
is stating that friction is proportional to the normal force, and the second
is stating that friction is independent of cross-sectional area.
Note that the second statement is counterintuitive; most of us would assume
that friction does depend upon the cross-sectional area.
He also made the observation that different materials
move with different ease. He surmised that this was a result of the
smoothness of the material in question; thus, smoother materials will have
smaller frictions. Da Vinci also made some quantitative statements
about friction. He stated that, "every frictional body has a resistance
of friction equal to one quarter of its weight." In contemporary
terms, the coefficient of friction is 0.25. By today's standards
we would say that wasn't even close, but for dirty materials that value
is fairly accurate. Leonardo Da Vinci did not publish his theories,
so he never got credit for his ideas. The only evidence of their
existence is in his vast collection of journals. These journals contain
a lot more than just writings about friction. In addition to his
famous artwork, their are drawings of submarines,
helicopters, and airplanes over 300 years before their inventions.
To learn more about the many other sides of Da Vinci check out these links.
Museum of Natural History
the Mona Lisa Smiling?.
Guillaume Amontons (1663-1705)
was an architect by training, but in those days this by no means meant
that he was limited to that one field. He designed one of the first
steam engines, and he was the first to state that a certain increase of
air's temperature produces a proportional increase in the air's pressure.
This led to him constructing the first gas thermometer. Because he
contracted a disease which left him deaf, Amontons developed a new system
of communication. He later used this new form of communication to
invent the first form of the telegraph, called the optical telegraph.
He is important in the field of tribology because he rediscovered the two
basic laws of friction that had been discovered by Leonardo Da Vinci, and
he also came up with an original set of theories. He believed that
friction was predominately a result of the work done to lift one surface
over the roughness of the other, or from the deforming or the wearing of
the other surface. For several centuries after Amontons' work, scientists
believed that friction was due to the roughnesses on the surfaces.
If would like to know more about Amontons' impact on science, please
look at these links.
Empirical Gas Laws and the Thermometer
Friction on an Atomic Level (disproving Amontons' Law)
Fig. 2: A drawing similar to this was in
Desagulier's paper demonstrating how lead balls adhere to each other.
Desagulier was another person who did much more than contribute to the
study of friction. He also made significant strides in the design
of the young steam engine. After several fatal accidents involving
exploding boilers, he invented the safety valve so that the pressure would
not get too high. He also designed his own engine which was very
successful for the time period at raising water from a well. This
is one of his sketches of this steam pump.
Desagulier's steam pump. Picture obtained
from the following site.
To learn more about the steam engine and his contributions to it, check
Growth of the Steam Engine.
John Theophilus Desagulier (1683-1744) had a significant
impact on the study of friction. He considered the argument of Amontons
about surface roughness and made the remark that "as surfaces are made
smoother they ought to slide more easily, yet it is found by experience
that the flat surfaces of metals or other bodies may be so far polished
as to increase friction." He attributes this to the adhesion between
the two surfaces. He conducted experiments with lead balls that illustrated
this idea. After cutting the two balls in order to expose fresh,
clean surfaces on each, Desagulier pushed the two surfaces together.
It then took 16 pounds of force to separate the lead balls. He believed
in this idea, but he was unable to explain the laws of friction relating
to adhesion because he could figure out how to justify the already proven
statement that friction is independent of cross-sectional area when adhesion
most definitely is.
Using the two principles shown above, Coulomb designed the first tribometer.
The tribometer is a device which measures the coefficient of friction of
a certain material. A high-tech version of Coulomb's tribometer is
still used today for research on material wear and lubrication.
Coulomb worked on friction, but he is best known to physicists for his
work on electricity and magnetism. He established experimentally the inverse
square law for the force between two charges, which became the basis of
Poisson's mathematical theory of magnetism. Coulomb also wrote on structural
analysis: the fracture of beams, the fracture of columns, the thrust of
arches. He also has a paper on the way sap circulates inside a tree.
He is also known to geotechnical engineers as the grandfather of soil mechanics.
If you are interested in finding out more about Coulomb, these are some
great pages to get you started.
Charles August Coulomb (1736-1806) also considered
the idea of adhesion, but he dismissed the idea because for it to be true
friction would have to be proportional to cross-sectional area. He
attributed friction to the work done to dragging one surface up the roughnesses
of the other surface. The following figures will help you to visualize
of soil mechanics