How do lubricants increase a machine's mechanical efficiency?

Explanation

**Step-by-Step Solution:**

1. **Understanding Mechanical Efficiency**: Mechanical efficiency of a machine is defined as the ratio of useful work output to the total work input. It is expressed as:
\[ \text{Efficiency} = \frac{\text{Output Work}}{\text{Input Work}} \times 100\% \]

2. **Role of Friction**: In any machine, there are moving parts that create friction. This friction opposes the motion of the moving parts, which results in some of the input work being converted into heat rather than useful work. This means that the output work is always less than the input work due to the energy lost in overcoming friction.
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