The major reason why a wet saw that is used in cutting optical crystals is constantly lubricated with oil is largely due to friction, lubrication decreases friction and minimises the increase of internal energy.
Friction is basically the coming together of two parts that rubs against each other, it causes heat and lead to deterioration of surfaces, it can also lead to wear and tear.
Although, despite it down side, friction is desire-able in most applications
e.g the brake system of vehicles, and if not for friction, we humans cannot walk without falling
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