What opposes motion between two surfaces in contact?

• A. Velocity
• B. Friction
• C. Mass
• D. Acceleration

Answer: (B)

Friction is the force that opposes motion between two surfaces in contact. It arises from rough surfaces and microscopic interactions, and it acts parallel to the contact area in a direction opposite to the motion or to the direction you’d push to start moving. There are two kinds: static friction, which resists starting to move up to a maximum limit, and kinetic friction, which acts when sliding occurs. The size of friction depends on how hard the surfaces are pressed together (the normal force) and how rough the surfaces are (the coefficient of friction). On a horizontal surface, the normal force is roughly equal to the weight, so friction grows with weight and surface roughness. In contrast, velocity is simply the speed and direction of motion, mass is the amount of matter, and acceleration is how speed changes—none of these are the force that opposes motion.