Despite its familiarity, friction is mystifying. When you try to shove a box across the floor, the amount of friction depends only on the weight of the box and how hard you push on it—not, weirdly, on the box's size or shape. The standard view is that friction occurs 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. But this explanation doesn't tell us how friction works on extremely smooth or inflexible surfaces. Now physicists Michael Marder of the University of Texas at Austin and Eric Gerde of Renaissance Technologies in Setauket, New York, have developed a mathematical model showing that in many cases cracks, not nubs, are the root cause of friction.
In Marder and Gerde's calculations, pushing on the box creates a small puckerlike crack that lifts off the floor and migrates along the bottom of the box, in much the way that a bump ripples across a rug when you try to massage it into place. "You'd think the surfaces would just be squished together. The natural world figures out clever ways to make things happen that we might have found difficult to predict," Marder says. Millions of these imperceptible cracks move along the boundary between the box and the floor, allowing the whole thing to slide. According to this model the amount of friction depends only on the physics of the cracks, not on the size of the contact area, just as the difficulty in moving a bump doesn't depend on the size of the rug.