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Flight is a balance of forces
Lift, weight, thrust and drag act on an aircraft.
The idea
Lift and drag are defined relative to airflow; weight acts toward Earth. A steady straight flight can have balanced forces without being stationary.
An example
An aircraft cruising at constant velocity still has forces acting on it.
Put it into practice
Distinguish balanced forces from an absence of forces.
Further reading: NASA · Glenn Research Center ↗
Test yourself: What can balanced forces produce?
Constant velocity in a straight line
Zero net force permits constant velocity; it does not require zero speed.
Explanation and illustrative examples by One Card Wiser. Source linked for further reading.
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