Which of the following is not a contact force?

• A. A push on a door.
• B. Gravity.
• C. Friction between two surfaces.
• D. Normal force.

Answer: (B)

Understanding which forces are contact forces versus non‑contact forces is the key idea here. Contact forces arise from objects physically touching, while non‑contact forces act at a distance through fields.

A push on a door is a contact force because it requires your hand pressing on the door. Friction between two surfaces is also a contact force, generated at the interface where the surfaces touch. The normal force is the support force a surface exerts perpendicular to its surface, again arising from contact.

Gravity, on the other hand, is a non‑contact force. It acts between masses without needing them to touch, through the gravitational field. So gravity is the force that does not require contact, making it not a contact force.