Which of the following is NOT an example of a contact force?

• A. Friction
• B. Tension
• C. Gravitational force
• D. Normal force

Answer: (C)

Gravitational force is considered a non-contact force because it acts at a distance without direct physical contact between the objects involved. For instance, the Earth exerts a gravitational pull on objects, such as an apple falling from a tree, without any physical interaction or contact. This force affects masses by attracting them towards each other, demonstrating that gravitational interaction can occur over distances.

In contrast, friction, tension, and the normal force are all examples of contact forces. They require direct physical interaction between surfaces or materials. Friction arises when two surfaces slide against each other; tension pertains to the force experienced by a string or rope when pulled; and the normal force is the support force exerted perpendicular to a surface in contact with an object. Each of these forces necessitates that the objects come into physical contact, thus differentiating them from gravitational force.