What is friction? The definition of friction, as listed in Webster’s is "A force that resists the relative motion or tendency to such motion of two bodies in contact", in Lehman’s terms, friction is what makes things hard to move.

It is now known that friction resists motion, does this mean that friction is bad? Without friction, it would be impossible to type this manuscript, impossible to drive to class each day, or even impossible to walk around the house. However, certain people would like to see the absence of friction. Engineers that deal with the friction within an engine are constantly trying to use new composite materials and more efficient oils to reduce the internal frictions of an engine, thus reducing wear and extending the life of the vehicle. So friction becomes a two-way street.

When it comes to friction, physicists are the guru’s. To a physicist, friction becomes more than a simple definition, friction becomes part of their life and their experiments. The idea of friction, oddly enough, is not influenced by the shape of an object, rather the only factors are the weight of the object (m*g) and how much of a force is applied (m*a), thus yielding the equation (µk=fk*N).

Friction, on the molecular level, has long been understood as "a result of the deforming and distorting of tiny nubs on the surfaces that are moving past one another, so the overall size of the contact area doesn’t matter" (Saunders). This concept is pretty much straight forward for rough or soft surfaces, however, a recent discovery by physicists Michael Marder of the University of Texas at Austin and Eric Gerde of Renaissance Technologies in Setauket, New York, have determined mathematically that cracks, not nubs, are the molecular cause of friction on very hard and smooth surfaces. Gerde and Marder determined through calculations that as a random object is pushed across a surface the object creates a "crack" that lifts off of the floor and follows along the path of the object. This concept is similar to the ripples that are produced and moved along a rug. When a person tries to push the ripples in the rug down, the ripple in the rug moves forward or backward and resists the force to be "squished together." Applying this concept to friction, the ripples (cracks) are what cause friction and therefore explain why the size of the contact area does not become a factor in the determination of friction.

Although the explanation of friction on extremely hard and smooth surfaces has changed slightly, every piece of information helps to better understand what it will take to reduce or increase friction in the future.

 

 

Saunders, Fenella "Friction is Such a Drag", Discover, February 2002, pg. 17

http://www.discover.com/recent_issue/index.html

(Scroll down to Feb. 2002 issue, then to R&D, then to "Friction is Such a Drag")