There are four fundamental forces involved and acting on the airplane at all times in flight.
Thrust is created by a propeller, jet engine, or rocket; and moves the aircraft in the direction of its motion. Thrust is generated through pulling air in and pushing air out in the opposite direction.
Drag is caused by friction and differences in air pressure. Think about the last time you stuck your hand out of a moving car. Air, as any other substance, has mass. The faster you move against that mass, the more resistance you will feel, and the more drag is created.
Weight is a result of Earth’s gravitational pull. The more mass the airplane has, the more lift will be necessary to overcome the weight of the airplane.
Lift is primarily created by the wings and must overcome weight for the aircraft to become airborne.
Watch the video above for a more thorough explanation.
IN UNACCELERATED FLIGHT, ALL FORCES ARE IN BALANCE (or equal). Even if the airplane is climbing or descending. The only thing that makes the forces unequal to each other is ACCELERATION!