Here's the rub

Andreas Heinrich and Markus Ternes at IBM's Almaden Research Center, in San Jose, California together with Franz Giessibl at the University of Regensburg, Germany, and their colleagues have for the first time measured the force needed to move a single atom across a surface. Their results now mean that friction can be studied at the atomic level as never before. The team used a modified atomic force microscope in ultrahigh vacuum at 5 K to measure the lateral and vertical forces exerted on a cobalt atom as the AFM tip dragged it across a metal surface.