Which of the following is NOT one of the six classical simple machines?

• A. Gear
• B. Lever
• C. Pulley
• D. Screw

Answer: (A)

Recognizing what counts as a simple machine: the six classical simple machines are the lever, wheel and axle, pulley, inclined plane, wedge, and screw. A gear is not listed among them because it isn’t a single, basic device that changes a force or direction by itself. A gear is a mechanism made of two or more wheels with teeth that mesh to transfer rotation and modify speed or torque; it requires multiple parts working together, so it’s considered a compound machine rather than a simple machine. The other options fit the definition of a simple machine: a lever uses a fulcrum to multiply input force, a pulley redirects force through a rope or belt, and a screw is essentially an inclined plane wrapped around a cylinder to convert rotational motion into linear motion. Therefore, gear is the best choice for not belonging to the six.