Aerodynamics - Four Forces

..	Weight - Lift - Thrust - Drag

Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight. In steady flight, the sum of these opposing forces is always zero. .. .. ..

	Weight
		The combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight is a force that pulls the aircraft downward because of the force of gravity. It opposes lift and acts vertically downward through the aircraft’s center of gravity (CG). The CG may be considered the point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any attitude.

You can read more about the concepts above in the Pilot’s Handbook of Aeronautical Knowledge. Chapter 4: Principles of Flight Chapter 5: Aerodynamics of Flight