Hoop stress is a stress in a pipe wall. It is represented by the forces inside the cylinder acting towards the circumference perpendicular to the length of the pipe.
Hoop stress is derived from Newton's first law of motion. Conceptually, the amount of internal pressure produced must be equal to the stress around the wall of the cylinder to maintain equilibrium and for objects to remain at rest. If forces inside a cylinder are simplified, another stress called longitudinal stress is derived. It is a force pushing against both ends of the pipe. These forces are measured in newtons per square meter (N/m*m) or pascals (Pa).