In which of the following situations is friction not useful?

Step 1: Analyze the question We need the situation where friction is not useful, meaning it is a nuisance, causing wear, heat and energy loss.

Step 2: Concept Friction is the force that opposes relative motion between two surfaces in contact.

• It is useful when we need grip, or when we need to slow something down or wear a surface away.
• It is not useful when it causes unwanted wear, heat and energy loss between moving parts. In that case we reduce it with lubricants, ball bearings or polishing.

Step 3: Check each option

• A. Grinding machine: Friction between the grinding wheel and the material wears the material down. This is the whole purpose of grinding, so friction is useful.
• B. Walking: Friction between the soles of our shoes and the ground gives grip. Without it we would slip, as on ice. Friction is useful.
• C. Application of brakes: Friction between the brake pads and the wheel or drum slows the vehicle. Friction is useful.
• D. Moving piston in a sleeve: A piston sliding in a cylinder or sleeve should move as freely as possible. Friction here causes wear, heat and loss of energy, which is why engines use lubricating oil. Friction is a nuisance, so it is not useful.

Step 4: Answer The correct option is D.

Tip: Ask whether the situation needs grip or stopping (friction useful: walking, brakes, grinding, striking a match) or smooth, continuous sliding (friction harmful: pistons, bearings, machine parts). In WASSCE, if you see moving machine parts, think "lubrication" and "friction is a disadvantage."