Looking at How Things Slip

To displace a body resting on a surface, a lateral force that is larger than the static friction force must be applied. The ratio of the friction force and the normal force (the coefficient of static friction) depends on some properties of the contact interface, such as the material type and the surface roughness, but not on the apparent contact area. These macroscopic laws of friction, formulated by Amontons at the end of the 17th century, remain under investigation today (1). An understanding of how these macroscopic forces originate in microscopic processes has application in materials and engineering and may even improve the modeling of earthquakes. On page 211 of this issue, Ben-David et al. (2) report careful measurements of local shear and normal stresses across a contact interface during frictional slip that show considerable nonuniformity. Although Amontons' laws apply globally to the entire system, they do not hold locally. The results of the study challenge the commonly held assumption that the lateral and normal forces are uniform across the contact interface and are thus likely to change the way we model friction.