What is the resistance between contacting surfaces called?

• A. Energy
• B. Power
• C. Momentum
• D. Friction

Answer: (D)

Friction is the resisting force that acts at the contact interface between two surfaces in contact, opposing their relative motion. It arises from microscopic roughness and adhesive interactions between surfaces and depends on how hard the surfaces press together (the normal force) as well as material and surface finish. In maintenance and equipment operation, friction shows up as energy loss, heat, and wear in moving parts such as bearings, gears, and conveyors. This makes friction the specific term for the resistance between contacting surfaces. The other choices describe different physical quantities—energy is the capacity to do work, power is the rate of doing work, and momentum is mass in motion—not the resistive interaction itself.