What is friction?

• A. Friction is the energy lost due to heat
• B. Friction is the force that opposes motion only in air
• C. Friction is the force that accelerates objects
• D. Resulting force that must be overcome when one body moves in contact with another

Answer: (D)

Friction is the resistive force that opposes the motion of one surface sliding (or trying to slide) against another surface in contact. It exists because real surfaces are not perfectly smooth; microscopic irregularities and some adhesive forces between surfaces make motion harder, so this force has to be overcome to move the object.

There are two kinds you should know: static friction, which holds the object in place up to a limit, and kinetic (sliding) friction, which opposes motion once the surfaces are sliding. The size of friction depends on how hard the surfaces are pressed together (the normal force) and the nature of the surfaces (roughness and materials). In equations, kinetic friction is roughly F = μk N, and static friction can vary up to F ≤ μs N.

Friction is not itself energy; it’s a force. The energy lost during motion shows up as heat and wear due to that force acting over a distance. It’s broader than just air resistance, which is a separate form of friction with a fluid.