What force does it take to push a single atom?

New York Times: IBM scientists have used the tuning fork in the atomic force microscope, which measures the interaction between the tip and the atom, to calculate the force needed to nudge one atom. About one-130-millionth of an ounce of force pushes a cobalt atom across a smooth, flat piece of platinum. Pushing the same atom along a copper surface is easier, just one-1,600-millionth of an ounce of force