For the four forces to be balanced, thrust must be equal to drag, and lift must equal weight. When this happens we say the airplane is cruising.
The force that resists an aircraft's motion through the air is called drag. Every component of the aircraft produces drag, including the engines. A mechanical force is a drag. It is created when a solid body interacts with and contacts a fluid (liquid or gas). It is not produced by a force field in the sense of a gravitational field or an electromagnetic field, in which two objects can interact without coming into touch with one another.
The solid body needs to be in touch with the fluid for drag to be produced. There won't be any drag if there is no fluid. The velocity difference between a solid object and a fluid causes drag.
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