What term describes a force that acts only when two objects are in contact?

A contact force is a force that acts only when two objects are in physical contact. Pushing a box, kicking a ball, and a table supporting a book are all examples of contact-force interactions.

Contact forces are often divided into components. The normal force acts perpendicular to the surfaces touching, while friction acts parallel to those surfaces and opposes relative motion or the tendency to move. Tension, spring force, and applied pushing forces are other common examples whose effects depend on physical interaction between objects.

The main mix-up is confusing contact forces with field forces. Gravity, electric force, and magnetic force can act across empty space, so they do not require the objects to touch. A book resting on a table illustrates both categories: Earth’s gravity pulls the book downward, while the table exerts an upward normal contact force.

At the microscopic level, ordinary contact is not perfectly solid-to-solid touching. Electromagnetic interactions between atoms, together with quantum effects such as the Pauli exclusion principle, prevent everyday materials from passing through one another.