Friction-free system
In a frictionless system , movements ( oscillation , rotation or sliding processes ) take place without any loss of usable energy (see exergy ).
Since such ideal systems do not exist in nature , the friction in the moving or oscillating system has to be modeled - which is usually done using differential equations . The amount of friction - the coefficient of friction - can be greatly reduced by good lubrication of the bearings . Smooth, hard surfaces on axles , wheels or other moving parts also reduce friction.
In smooth systems, processes are reversible .