1. Lift and Weight (Gravity):

* Lift is the upward force generated by the wings, which counteracts the force of gravity pulling the plane down.

* Weight is the force of gravity acting on the plane's mass.

2. Thrust and Drag:

* Thrust is the forward force generated by the engines, propelling the plane through the air.

* Drag is the resistance to motion caused by air friction, acting in the opposite direction of motion.

For stable flight, these pairs of forces must be equal and opposite, creating a state of equilibrium.