When a book rests on a table, which statement correctly describes the normal force?

• A. Perpendicular to the surface
• B. Parallel to the surface
• C. In the same direction as gravity
• D. Always equal to mg

Answer: (A)

Normal force is the contact force that acts perpendicular to the surface at the point of contact. For a book resting on a table, the table pushes on the book along a line normal to the surface, pointing upward. Gravity pulls the book downward with magnitude mg. On a horizontal table with no other vertical forces, these vertical forces balance, so the normal force has the same magnitude as the weight in this simple case. The crucial feature is that the normal force is perpendicular to the surface, not along it, not in the direction of gravity, and not automatically equal to mg in every situation. The other statements describe friction (parallel to the surface), gravity’s direction, or a universal equality to mg, which aren’t generally correct.