CBSE Class 11-science Answered

If the surfaces are extremelyy smooth and they are brought together in vacuum( so that they stay clean) they cannot be made to slide over each other. It is a kind of cold weld. Many atoms of one surface are in contact with many atoms of the other surface so they join together instantly forming a single piece of metal.

However if  air is present the atom to atom contact is less and the blocks stick firmly to each other and require considerable force to be separated. But generally so much atom to atom contact is not possible and even a highly polished metal suface is not flat if seen on the atomic scale. Everyday objects that we consider have layers of oxides and many contaminants that prevent cold welding.