Perpetual motion machines can never be built because it is not possible to eliminate what?

The concept of perpetual motion machines is grounded in the laws of thermodynamics, particularly the first and second laws. A machine that operates perpetually without an external energy input would need to create more energy than it consumes, which violates these fundamental laws.

Friction plays a critical role in this discussion. Friction is a force that opposes motion, causing energy to be lost as heat. Any real machine experiences friction, meaning it inevitably loses energy over time, so it's impossible for it to run indefinitely without an external energy source. Similarly, matter cannot simply be eliminated, as it's a fundamental part of physical systems, and energy cannot be created or destroyed, only transformed.

Inertia, while an important principle of motion, does not directly relate to the generation of energy or the maintenance of motion without an energy input. The inability to eliminate friction from machines is what directly contributes to their failure to achieve perpetual motion. This makes it clear that perpetual motion machines cannot be constructed due to the impossibility of overcoming these physical realities.