What is the force that must be overcome when two bodies in contact move?

• A. Friction
• B. Gravity
• C. Drag
• D. Lift

Answer: (A)

Friction is the force that must be overcome when two surfaces in contact move relative to each other. Surfaces aren’t perfectly smooth; tiny irregularities and intermolecular forces cause resistance that you must overcome to start or keep moving. There are two kinds: static friction, which must be overcome to start motion, and kinetic friction, which resists motion once sliding has begun. The amount of friction depends on how hard the surfaces press together (the normal force) and the materials’ roughness, commonly summarized by F_f = μ F_N. Gravity increases the normal force and thus friction; drag and lift are fluid-related forces, not the primary resistive force between solid contact surfaces.