What is friction?

• A. The energy produced when surfaces collide.
• B. The gravitational force between two bodies.
• C. The resulting force that must be overcome when one body moves in contact with another body.
• D. The resistance to motion of objects in a vacuum.

Answer: (C)

Friction is the resistive force that opposes the relative motion between two surfaces that are in contact. It’s the force you must overcome to start moving or to keep an object sliding across a surface, arising from microscopic interactions at the contact points and the pressure pressing the surfaces together. The option that best matches this idea describes friction as the resulting force you have to overcome when one body moves in contact with another body.

The other ideas don’t fit: energy from surfaces colliding describes impact energy, not the resisting force of friction; gravity is a separate force due to mass; and framing friction as a resistance to motion in a vacuum misattributes the cause, since friction comes from contact between surfaces rather than the presence of air or a vacuum.