Which simple machine consists of two circular objects that rotate about a common axis?

The correct answer is the wheel and axle, which is indeed a simple machine that consists of two circular objects that rotate about a common axis. In this configuration, the wheel is the larger circular component, while the axle is the smaller, central rod or shaft. When the wheel turns around the axle, it allows for the transmission of force and movement, making it easier to apply effort and reducing friction.

The wheel and axle setup is commonly used in various applications, such as in vehicles, where the wheels rotate around the axles to facilitate motion. This system effectively converts rotational motion into linear motion, providing mechanical advantage in lifting or moving objects.

Other options present different types of simple machines that do not fit this description. An inclined plane aids in raising heavy objects by reducing the required effort but does not involve rotation. A pulley consists of a wheel that changes the direction of force, typically using a rope or cable, rather than two rotating objects about a common axis as seen in the wheel and axle. The screw is a different kind of machine, utilizing a helical design to convert rotational force into linear movement, not defined by two circular objects rotating around a common point. Thus, the wheel and axle distinctly fits the criteria stated in the question.