A machine cannot achieve 100% efficiency in the real world primarily due to the following reasons:
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Friction: Machines are subjected to various forms of friction between moving parts. This friction converts some of the useful mechanical energy into heat, which is generally lost to the environment, reducing the overall efficiency.
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Energy Losses: Aside from friction, there are other forms of energy losses such as air resistance, thermal losses, and sound. These contribute to the decrease in the proportion of input energy that is converted into useful work.
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Mechanical Limitations: All materials and components have inherent limitations, such as wear and tear, deformation, and material fatigue, which can lead to energy losses and inefficiencies over time.
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Heat Generation: According to the laws of thermodynamics, energy transformations are not 100% efficient. Whenever energy is converted from one form to another, some energy is typically lost as heat.
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Real-world Conditions: Ideal conditions often differ significantly from actual working conditions. Variations in material properties, wear, and the environment can all impact a machine's efficiency.
Thus, the correct statement from your options would generally relate to friction and other energy losses, leading to less than 100% efficiency in real-world machines. The option "machines must overcome forces of friction" captures one of the primary reasons for this inefficiency. The statement about output work always being more than input work is inaccurate, as it contradicts the definition of efficiency, which is typically the ratio of output work to input work expressed as a percentage.