What must thrust overcome for an aircraft to accelerate?

Thrust must overcome drag for an aircraft to accelerate. Drag is the resistance force that opposes an aircraft's motion through the air. It is caused by the friction of air molecules as they interact with the aircraft's surfaces and is influenced by factors such as the shape of the aircraft, its speed, and the density of the air.

When an aircraft is in flight, for it to increase its speed or accelerate, the thrust produced by the engines must exceed the drag acting against it. If thrust is greater than drag, the result is acceleration; if it is equal, the aircraft maintains speed; and if it is less, the aircraft will decelerate.

In contrast, other forces such as weight and gravity are significant for an aircraft's overall flight mechanics, particularly during climb or descent, but they do not directly impact the ability to accelerate horizontally in the manner that drag does. Lift is essential for keeping the aircraft airborne but does not directly factor into horizontal acceleration either. Lastly, roll is a maneuvering action relating to the aircraft's orientation, not a force that must be overcome for straight-line acceleration.