Weight, resistance and terminal motion

Weight is W= mg. Near Earth use g=kg9.8N​ unless otherwise supplied. In a lift, choose a vertical axis and apply Fnet = ma. A scale measures support force, which need not equal weight when accelerating. Free-falling objects still have weight even though their support force can be zero.

Friction and drag oppose relative motion. During a fall, drag often grows with speed until it balances weight. Terminal speed then has zero net force and zero acceleration, but non-zero velocity. Before balance, changing drag means changing acceleration, so constant-acceleration equations are generally unsuitable. At fixed mass, a larger net force produces larger acceleration; at fixed force, a larger mass produces smaller acceleration.

Watch out: Weight still acts. Drag balances it, giving zero net force and zero acceleration while velocity remains non-zero.