What assembly moves inside the barrel-shaped body of a cylinder?

The assembly that moves inside the barrel-shaped body of a cylinder is the piston. In a hydraulic or pneumatic cylinder, the piston is a cylindrical component that reciprocates within the barrel when hydraulic or pneumatic pressure is applied. This movement converts hydraulic or pneumatic energy into mechanical work, enabling various machinery and equipment to operate efficiently.

Pistons are essential for creating the force necessary to perform work, making them a fundamental element in applications like engine cylinders, hydraulic pumps, and actuators. The design of the piston allows it to create a seal against the walls of the cylinder, ensuring that pressure does not leak and that the movement is effective.

The other options refer to different components that serve distinct functions. Poppets and springs are more often associated with valves and control mechanisms and do not move within the barrel itself. An element usually refers to a specific part of a system but does not denote movement inside the cylinder. Vanes are used in rotary mechanisms and do not operate in a linear movement like a piston in a cylinder.