What is a primary reason why machines cannot achieve 100% efficiency?

Machines cannot achieve 100% efficiency primarily due to the effect of friction. Friction arises when moving parts of a machine interact, which leads to energy loss as heat rather than maintaining it for useful work. This unavoidable resistance diminishes the overall energy that can be applied to perform the intended function of the machine.

In practical terms, every machine involves contact between its components, whether gears, bearings, or other moving parts. These interactions generate heat that dissipates energy, reducing the machine's efficacy. As a result, while machines can be designed to minimize friction through better materials and engineering practices, they can never fully eliminate it, thus preventing 100% energy conversion from input to output.

The other factors, while relevant in broader contexts, are not the primary reason machines fall short of perfect efficiency. For instance, the inability to create energy is a fundamental principle of physics but does not specifically account for efficiency losses within a machine's operation as friction does.