What is the force perpendicular to a surface that prevents an object from falling through the surface?

• A. Normal Force
• B. Tension
• C. Work
• D. Power

Answer: (A)

The force in question is the normal force. It’s a contact force that acts perpendicular to the surface and prevents the object from passing through the surface. When gravity pulls the object downward, the surface pushes back upward with a force that is perpendicular to the surface. This normal force provides the support you feel when something rests on a table and, together with gravity, determines whether the object stays put or moves. By Newton’s third law, the object presses on the surface with an equal and opposite force, but the surface’s push on the object is what stops penetration. On a horizontal surface with no other vertical forces, the normal force balances the weight.

Tension is a pulling force along a rope or string, not something that generally acts perpendicular to a surface. Work is a measure of energy transfer that occurs when a force moves an object, and power is the rate at which work is done. Neither of these describes a force that prevents an object from falling through a surface.