Final answer:
The thrust force is the main external force affecting a helicopter's upward motion due to the action and reaction between the helicopter blades and the air. This force is in accordance with Newton's Third Law of motion.
Step-by-step explanation:
The force that affects the helicopter's motion while hovering is the thrust force. This is due to Newton's Third Law, which states that for every action, there is an equal and opposite reaction. The helicopter blades push air downwards (action), and as a result, the air pushes back upwards against the helicopter (reaction), creating lift. This upward force is external to the helicopter and contrary to the gravitational force pulling it down.
It's important to note that while the gravitational force is also affecting the helicopter by pulling it towards Earth, the specific upward movement is due to the thrust force generated by the action of the helicopter blades on the air. This is analogous to birds flapping their wings and airplanes utilizing their wings to alter the airflow and create lift. Both birds and airplanes apply a force to the air, and the air applies an equal force in return, which allows for flight.