Concept:Friction is a force that opposes relative motion between two surfaces in contact. It is useful only when motion is deliberately controlled or enhanced, but it is harmful when it wastes energy and causes wear in moving machine parts.
Explanation:In a grinding machine, friction between the abrasive wheel and the workpiece is necessary to remove material.
While walking, friction between the foot and the ground prevents slipping and provides grip.
When brakes are applied, friction between the brake pads and the wheel slows down or stops the vehicle.
However, in a moving piston inside a sleeve, friction is not useful because it opposes smooth motion, causes wear and heating, and reduces efficiency.
Therefore, lubricants are used to reduce unwanted friction in such engine parts.
Answer:D. Moving piston in a sleeve