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How does friction change mechanical energy into thermal energy?

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Study how a series of gears in a waterwheel translates a stream's energy to a millstone — energy conversion Before the Industrial Revolution, power came from three main sources: humans, draft animals, and water.
The ingenuity people used in harnessing waterpower can be seen in this medieval-style mill. The waterwheel is turned by a stream and is connected to a shaft that leads into the building. At the other end of the shaft is a gear. The connection of a series of gears translates the power from the stream to a shaft that drives a millstone, which grinds flour from grain.
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In the context of mechanical systems, friction acts as a force that resists the motion of objects, effectively degrading mechanical energy into thermal energy.123

When an object is in motion, it possesses kinetic energy. As it slides or rolls against another surface, frictional forces—arising from microscopic irregularities and adhesion between surfaces—oppose this motion. To overcome this resistance, work must be performed, which results in the dissipation of the object's kinetic energy. This energy is not destroyed, as dictated by the first law of thermodynamics, but is instead converted into thermal energy, which manifests as heat.123

It is important to note that this process is fundamentally irreversible. According to the second law of thermodynamics, while mechanical energy can be converted into heat through friction, that heat cannot be entirely converted back into useful mechanical energy. This degradation of energy is a critical consideration in all energy-conversion processes, as it represents a loss of efficiency in mechanical systems.2