Why contact still resists motion without air
A vacuum removes the gas molecules between two objects, but it does not remove the objects themselves. Press them together and their surfaces still meet at tiny spots. Even a surface that looks polished has microscopic high points, called asperities, that carry much of the load.
As one surface slides, those contact points deform, scrape past one another, or form and break tiny adhesive bonds. The motion takes energy, which is why a force is needed to keep the surfaces sliding. That resistance is friction; it comes from the interaction between the solids, not from air trapped between them.
What the vacuum changes
Air can add its own drag to a moving object, but that is separate from friction at a solid-on-solid contact. Pumping air away reduces that contribution while leaving the surfaces pressed together. The amount of friction depends on the materials, surface condition, and force holding them in contact—not simply on whether the surrounding space contains air.
A vacuum can also change what happens at the contact. In ordinary air, thin films of water or other contaminants often coat surfaces. Removing those films can alter friction, and exceptionally clean metal surfaces may stick strongly where they touch. So a vacuum does not guarantee the same friction as in air; it simply cannot make contacting solids frictionless by removing the gas around them.
The practical distinction is simple: a moving object in a vacuum avoids air resistance, but a sliding bearing or tool can still lose energy wherever its surfaces rub.
