Two great physical forces control human interaction with the Universe, gravity and friction. The former has been studied at length by the great men of science, Newton, Einstein, Hawking, Berle. The latter has been largely ignored. Friction is what wears out machines, and makes them inefficient. The proper application makes other machines work. Therefore, friction has been studied by mechanical engineers. Without friction between shoes and the floor walking would be nearly impossible, we would need sharp spikes on our feet, to pierce the surface in order to have traction, and Mom would have really ugly floors. The question is: How much friction is enough?
Coefficient of friction (COF) is a ratio that describes the available friction as a proportion of the normal force (weight) pressing the surfaces together. So if it takes 50 pounds to begin to slide a 100 pound weight across a floor we say the COF is 1/2 or 0.5. This is called the static COF. There are no units for COF, it is a ratio, like a batting average. Once sliding has begun, COF is usually lower. We call this the dynamic COF. Dry friction must be defined in terms of both materials involved, e.g., tile floor and leather shoe sole, etc. There is no such thing as COF for one or the other, although some materials happen to usually contribute to a high or low value. Wet ice in almost any combination tends to have low COF, thus studded snow tires. Really rough concrete tends to go with high COF, unless it’s covered with ice. COF is independent of area, temperature, humidity, and velocity in High School Physics. As a concept it is good enough. It works for ordinary materials like wood, stone, and metal under a range of conditions. In particular, when there is no deformation.
There are other types of friction, for example, fluid internal friction called viscosity. Friction between polymers, like rubber, wax, leather and many soft plastics is called viscoelastic friction, and only follows the High School Physics model approximately, about as well as the high school football team followed physics. Wet surfaces fall into several different classes that hardly follow the High School Physics model at all: they are classified according to the type of lubrication: marginal, hydrodynamic and hydrostatic. Of these, viscosity and hydrostatic lubrication can usually be ignored in slip and fall reconstruction, unless the plaintiff slipped on molasses or STP.
There are more ways to measure friction than there are kinds of friction. This is because mechanical engineers need to know how friction affects machines under exacting conditions. Only a few test techniques are appropriate for most human locomotion, but ASTM lists at least 14 test protocols for COF of walking surfaces! Every industry from carpet to wax has its own test. The better tests are non-subjective, i.e., set it up, activate a control, read the meter, report the result. No further manipulation should be needed. Unfortunately they may not be meaningful for every situation.
Got a case involving friction? Don’t let it slip! Call a Forensic Consultant.
This article written by Kenneth Obenski, PEwas publishedin The San Diego Daily Transcript on May 3, 2006 as part of the Forensic Consultants Association Newsletter. Ken Obenski is a Mechanical Engineer who specializes in accident reconstruction for John Fiske Brown, Assoc., www.fiskebrown.com.
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