Which action would decrease friction between two surfaces?

• A. Apply lubricant
• B. Increase normal force
• C. Roughen surfaces
• D. Use a friction-enhancing material

Answer: (A)

Friction comes from the roughness of surfaces interacting and the forces that try to pull them together. A lubricant creates a thin film between the surfaces, so they don’t press directly against each other. That film reduces the real contact area and the interlocking of microscopic peaks, making it easier for one surface to slide over the other. In other words, the sliding resistance drops because the surfaces aren’t rubbing directly against one another as hard.

Increasing the normal force pushes surfaces together more tightly, which usually increases friction. Roughening surfaces increases irregularities that catch and resist movement, raising friction. Using a material that increases friction makes sliding harder, not easier. So putting in a lubricant is the action that lowers friction the most.