Since ancient times we have struggled to overcome
or tried to manipulate the forces of friction. The drawing
above shows a team of 172 slaves pulling a 60 ton stone statue.
One slave is reducing friction by pouring slippery animal
fats in front of the sled on which the statue rests. This
attempt to overcome friction is nearly 4000 years old. An
even earlier attempt at overcoming friction was likely the
driving force or ‘necessity’ that mothered the invention of
the wheel. In spite of how long this struggle has been going
on we are perhaps more than ever trying to understand and
control friction today.
Have you ever thought about the importance of
the negative effects of friction? They have staggering economic
consequences. It has recently been estimated that the cost
of friction in the United States is about $420 billion or
6% of the gross national product. This estimate only accounts
for losses that could be eliminated with current technologies.
One can only imagine how much might be saved with new technologies.
Rubber
Meets The Road
No one can argue the great impact that the invention
of the wheel had on reducing friction in transportation but
where the ‘rubber meets the road’ friction is still a big
factor.
Driving a car is expensive and most of this
expense is due to friction. Cars use the chemical energy stored
in motor fuel very inefficiently. This waste of energy is
largely due to the effects of friction. The fluid friction[glossary]
of air resistance, the friction between tires and the road
and the friction between engine parts are all part of this
problem. Friction also causes wear in a car’s moving parts.
Even if an engine is properly lubricated it will not last
forever and forgetting to change the oil will destroy an engine
in a hurry.
1994 Ford Probe GT 6 cyl. engine
So if we have convinced you that understanding
friction is indeed important where do we start?
Classical
Laws
Although the Egyptians were struggling with
friction over 4000 years ago, the formal written study of
friction did not begin until the fifteenth century with the
pioneering work of Leonardo da Vinci. About 200 years later
in 1699 a Frenchman named Guillaume Amontons rediscovered
what da Vinci had observed and formulated what we now call
the Classical Laws of friction:
Law #1:
The force of friction is directly proportional
to load.
Law #2:
The force of friction is independent of the
apparent area of contact.
Leonardo da Vinci, a self portrait
a) experiment to determine friction on
horizontal and inclined planes.
b)experiment to determine the affect of
apparent contact area of friction.
Simply stated, these rules say that a heavier
brick is harder to slide than a light one and the force necessary
to slide it is the same whether the brick is placed on end
or on a side.
Corollaries
Two other simple corollaries of friction are
often taught in physics classes:
Corollary #1:
The force necessary to set an object in motion
is usually larger than the force necessary to keep it in motion.
Corollary #2:
Within a certain range, the force of friction
is independent of velocity.
Space Shuttle
A snowboarder
A mountain climber
These rules are often useful tools for solving
friction problems but they fall short when describing most
frictional systems.
Tribology
Nearly all real frictional systems of interest
are much more complex than a hard dry block sliding on a hard
dry surface, yet many students of science and engineering
do not pursue an understanding of friction beyond this level.
The study of friction, also known as tribology, is a vibrant
field in which even basic theories are currently being disputed
and new discoveries are being made.
Friction
Defined
We all have some idea of what friction is. We
know that when there’s not enough friction we slip on the
ice or spin our car’s tires. We know that rubbing our hands
together makes them warmer or rubbing them with lotion makes
them slippery. This kind of first hand experience gives us
an intuitive understanding of friction, but what makes friction
so complex that were are still trying to understand it after
more than 500 years of study?
To answer this question it is first necessary
to ask- what is friction? If you were asked to define friction,
what would you say? Webster’s dictionary says friction is
‘a force acting against the direction of motion for two objects
in contact sliding across one another.’ This definition sounds
reasonable but does it really answer the question of what
is friction? To answer this question we must ask questions
like: What gives rise to friction? Why does it produce heat
and sound? Why and how do different materials affect friction?
Atomic
Force Microscopy
The reason these questions have not been fully
answered is that the tools to explore them have not been available
until recently. These questions must in part be answered on
a very small scale- the nanoscale. The interactions between
the molecules of surfaces are the key to understanding friction.
Topometrix Atomic Force Microscope
It was not until after WWII that theories of
the solid state and powerful new nanotools such as electron
beam probes were brought to bear on the subject. Many fundamental
questions about friction were answered in that time and there
was not a great deal of research in these fundamental questions
from about 1960 to the mid 1980s. However, in the last decade
or so, interest has been rekindled with tools like the scanning
probe microscope. Friction force microscopy, a mode of scanning
probe microscopy, has become a valuable tool for the study
of these interactions.
Deflection and Friction force images taken
with DI AFM
