What are two different sized circular objects that turn as one called?

The term for two different sized circular objects that turn as one is a wheel and axle. In a wheel and axle system, one circular object, or wheel, is fixed to a cylindrical object, the axle. When the wheel turns, it causes the axle to turn as well. This system is commonly used in various applications, such as vehicles, where the larger wheel can provide the motion and the smaller axle can accommodate a different functional purpose, like steering or propelling.

This mechanism is efficient because it allows for the transfer of force and motion between different sizes of circular objects, which can improve speed or torque depending on the size ratio. The wheel's larger radius allows for less effort to be applied for greater movement at the axle, highlighting the mechanical advantage this system provides.

Other options do not accurately describe the function of two circular objects turning together. A lever is a simple machine that allows a force to be amplified through a pivot point, while a pulley involves a wheel on an axle or shaft that helps in changing the direction of a force. A gear system includes multiple gears that mesh together, typically used for mechanical advantage and changing rotational speeds, but it is not simply two circular objects turning together in the manner described.