Which force opposes motion between two surfaces that are in contact?

• A. Friction
• B. Normal Force
• C. Work
• D. Power

Answer: (A)

Friction is the force that opposes motion between surfaces that are in contact. It acts parallel to the contact surface and points opposite to the direction of relative motion or the motion you’re trying to cause. This is why pushing a book across a table feels like you’re pushing against a resisting force—the friction is trying to keep the book from sliding. There are two kinds: static friction, which prevents motion up to a limit, and kinetic friction, which acts when surfaces slide against each other. The normal force, which is perpendicular to the surface, supports weight but does not oppose motion along the surface. Work and power describe energy transfer and rate of energy transfer, not a force that directly opposes motion.