Final answer:
The phenomenon of a helicopter sinking into its own downwash is known as vortex ring state or settling with power, and it is the external forces from the air interacting with the rotor blades that affect the helicopter's motion.
Step-by-step explanation:
The Phenomenon Affecting a Helicopter's Motion
The phenomenon of a helicopter sinking into its own downwash is known as vortex ring state or settling with power. This situation occurs when a helicopter descends too quickly into the air that it has just pushed downwards. Now, regarding the specifics of which force affects a helicopter's motion, recall that the movement of a helicopter pushing air down (to generate lift) creates an equal and opposite reaction that pushes the helicopter up, according to Newton's Third Law of Motion. This upward force is an external force acting on the helicopter, because it results from interactions between the helicopter's rotor blades and the surrounding air, which is outside of the system defined by the helicopter itself.
In physics, understanding how objects like helicopters are affected by forces is crucial. For instance, the downwash generated by the rotor blades is part of the helicopter's interaction with the surrounding environment, and the response to this action affects the flight dynamics. In simple terms, the helicopter's motion is influenced not by internal forces within the helicopter, but rather by external forces from the air that the helicopter interacts with during flight.