Which force acts parallel to a surface and opposes sliding between surfaces in contact?

• A. Buoyant Force
• B. Normal Force
• C. Friction
• D. Inertia

Answer: (C)

Friction is the force that acts parallel to the contact surface and opposes sliding between the surfaces in contact. It arises from the microscopic roughness of surfaces and the interactions at their contact, and it always acts to oppose the relative motion or the tendency to move. For example, when you push a book across a table, friction points opposite your push, resisting the motion. If you push hard enough, the book starts to slide and kinetic friction takes over, which is typically proportional to the normal force through the coefficient of friction.

This is different from buoyant force, which acts upward perpendicular to the surface of a fluid; the normal force, which pushes perpendicular to the contact surface; and inertia, which is the tendency of an object to resist changes in its motion rather than a force acting along the surface. Therefore, the force described is friction.