All motors have a rotor (the spinning part) and a stator (the stationary part). In linear motors the part the slides along can be thought of as the 'rotor', and the non-moving base, as the 'stator'.
The motor spins by causing like poles on the stator and rotor to oppose each other (different motors - AC, DC, brushless - do this in different ways).
For maximum efficiency it is desirable to build the rotor and stator in such a way that there is very little space between the two.
This allows a high degree of magnetic flux linkage/concentration between the rotor and stator. Between the rotor and stator is a small air gap.
Air, like all fluids, has viscosity (a sort of 'sticky ness ' , if you will). Viscous force appears between two sliding objects which have a liquid in between them.
The closer the sliding surfaces, the greater is the viscous force. Viscous force also depends on the area of the surfaces, their relative velocity, and the liquid's properties. For a detailed derivation please refer to any text on fluid dynamics.
Another important motor characteristic is back EMF (BEMF), which is induced in motor coils due to magnetic interaction.
The faster a rotor spins, the greater the magnitude od back EMF. Back EMF, as its name suggests, has a polarity opposite to the applied (battery/power source) EMF.
It is the back EMF which causes a reduction in motor current, since it partially cancels out the applied EMF : (Applied EMF-Back EMF)=(Effective EMF). Since the coils in the motor have unchanging resistance, a lower apparent EMF imples lower current.