Final answer:
The tail rotor on a helicopter is used to stabilize the aircraft, counteracting the rotational torque of the main blades due to Newton's third law of motion, ensuring the helicopter does not spin in the direction opposite to the main rotor.
Step-by-step explanation:
Purpose of the Small Propeller on Helicopters
The purpose of the small propeller at the back of a helicopter, also known as the tail rotor, is to stabilize the helicopter. Without the tail rotor, the helicopter's body would rotate in the opposite direction of the main lifting blades. This effect is a result of Newton's third law of motion and the conservation of angular momentum. The reaction force to the rotating blades would cause the body to spin in the opposite direction if not counteracted by the tail rotor's thrust.
This phenomenon is similar to the effect observed when a figure skater pulls in their arms and spins faster. The movement of the skater's arms affects the moment of inertia and the angular velocity of the spin. Similarly, helicopters with dual lifting blades that rotate in opposite directions do not require a tail rotor, avoiding this counter-rotational effect altogether.
To understand this in terms of Newton's third law, every action has an equal and opposite reaction. The main rotor provides a torque that would otherwise cause the body to spin, but the tail rotor exerts a counter-torque, preventing this unwanted spin and stabilizing the aircraft.