What do you call a force that opposes the motion of one surface as it moves across another?

Answer 1

This is one form of the property called friction. It is not a kinetic force, but opposes a force. It is caused by the interaction between molecules of the interacting surfaces. When motion occurs between surfaces, friction may convert mechanical energy to thermal energy (heating).

For some surfaces, notably liquids, it can be much less than others, notably rough solids. There are different types of friction, and rolling friction is usually less than sliding friction. Lubricants are used to reduce friction by adding a layer of smoother or liquid material between surfaces.