Final answer:
Hydraulic system 3 pressure is cut off from the main and nose gear selector valves to ensure priority to the primary flight controls. This safety measure guarantees that critical flight controls maintain operation during low-pressure scenarios, utilizing the principle that force multiplied by area equals pressure to operate heavy aircraft components efficiently.
Step-by-step explanation:
Hydraulic system 3 pressure is cut off from the main and nose gear selector valves if system 3 pressure drops below 2100 psi to ensure priority to the primary flight controls. This is because maintaining the operation of primary flight controls is crucial for the safety and control of the aircraft. If hydraulic pressure is too low, it might not be sufficient to operate all systems, so the system is designed to ensure that critical flight controls maintain pressure. By cutting off supply to less critical systems like the landing gear, it prevents a situation where there could be insufficient pressure to control the airplane, which would be a serious safety concern.
Hydraulic systems operate on the principle that force multiplied by area equals pressure. In an aircraft, hydraulic systems are designed to convert a small input force into a much larger output force. This allows for heavy components, such as the landing gear, to be moved with relatively small force inputs from the pilot or the aircraft's automated systems.