The force that opposes motion between two surfaces that are in contact is called?

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

Answer: (C)

The main idea is understanding the force that resists motion when two surfaces touch. That force is friction. Friction acts parallel to the contact surface and points opposite to the direction of movement. It happens because surface roughness and microscopic interactions between the contacting molecules grab and slide past each other, and it can be static (when there’s no motion yet) or kinetic (when the surfaces are sliding). The amount of friction depends on how hard the surfaces are pressed together (the normal force) and how rough or smooth the materials are (the coefficient of friction). This is why pushing harder or using rougher surfaces generally increases friction, making it harder to start or continue moving.

Gravity is the pull toward the center of the Earth, mass is the amount of matter in an object, and inertia is the resistance to changes in motion. None of these describe the specific resistance between two surfaces in contact like friction does.