I was glad to know why the so-called “eternal motion” could not yet be invented?

Answer

The perpetual motion or perpetuum mobile is provably impossible. The first law of thermodynamics (law of conservation of energy) gets in the way here.

An object can be set in motion and continue to move (the uniform rectilinear motion), but only if no force acts on it. As you yourself correctly point out, this is never the case in practice: frictional forces, gravity, etc., always have an effect. Moreover, the law of conservation of energy also states that if you somehow ‘tap’ energy from a moving object, that object loses energy and thus slows down. There is no way around. So even if you get an object moving with a minimum of external influences (eg a space probe), you can only look at it and do nothing else with it.

Perpetuum mobiles have not yet been invented and will never be invented, unless one can ever circumvent that first law of thermodynamics. But it is unlikely that that will ever happen. For an accessible explanation about the law on conservation of energy, please visit http://nl.wikipedia.org/wiki/Wet_van_behoud_van_energie ; or about perpetuum mobiles, http://nl.wikipedia.org/wiki/Perpetuum_mobile.