What term describes the resistance encountered when two surfaces slide past each other?

• A. Momentum
• B. Gravity
• C. Friction
• D. Inertia

Answer: (C)

Friction is the resistance that occurs when two surfaces slide past one another. It arises from the tiny irregularities and interactions at the contact points between the surfaces, and it acts parallel to the interface in the direction opposite to motion. The amount of friction depends on the normal force pressing the surfaces together and the materials involved, often summarized by the coefficient of friction. There are two main kinds: static friction, which must be overcome to start sliding, and kinetic (sliding) friction, which acts once sliding has begun and is typically a bit smaller than static friction. Friction can cause heat and wear, and lubricants reduce it by creating a slippery film between surfaces. For example, when pushing a heavy object, you first overcome static friction to start moving, then continue to overcome kinetic friction as it slides.