What is friction?

• A. The force of gravity.
• B. The force between air and surfaces.
• C. The force between materials as they pass each other.
• D. Energy lost as heat.

Answer: (C)

Friction is the force that opposes sliding between two surfaces that are in contact. It arises from the roughness and microscopic adhesion at the interface, and it acts parallel to the contact surface, opposite to the direction of motion or the tendency to move. There are two main kinds: static friction, which prevents motion up to a maximum limit, and kinetic (sliding) friction, which acts once the surfaces are moving relative to each other and is typically smaller in magnitude. The strength of friction depends on how hard the surfaces press together (the normal force) and on the materials and surface textures, summarized by the coefficient of friction. Friction is what lets you walk without slipping, lets brakes slow a car, and makes gears resist motion. Note that gravity is a separate force that can press surfaces together, and air resistance is a different force from a fluid. Energy lost as heat is a consequence of friction doing work, not the definition of friction itself.