Which component in a mechanical system serves to change rotational direction or speed?

The correct answer, a gear, plays a crucial role in mechanical systems by enabling the change of rotational direction or speed. Gears are toothed wheels that mesh with one another and transfer motion between different parts of a machine. When one gear turns, it causes another gear to rotate in the opposite direction if they are directly engaged.

Moreover, gears can differ in size. A larger gear turned by a smaller gear will change the speed of rotation, allowing for modifications in speed and torque. This capability is essential in various applications, from simple devices to complex machinery, as it allows for precise control of movement and is fundamental to the operation of engines, clocks, and even bicycles.

In contrast, other components in the list serve different functions. A lever is designed to amplify force but does not inherently change rotational direction or speed. An axle is a central shaft for rotating wheels or gears but does not directly influence rotation itself. A beam typically provides structural support or stability and does not change rotational characteristics in mechanical systems.