Because lift depends on speed squared, if a plane slows from 100 knots to 90 knots with the same wing shape, angle of attack, and air density, what happens to its lift?

Answer: Drops by about one fifth

Drops by about one tenth — Close, but this is the linear guess: 90 is 10 percent below 100. Lift does not scale linearly with speed in the basic lift equation; it scales with speed squared. That turns a modest-looking speed loss into a bigger aerodynamic loss, which is why pilots guard airspeed so carefully near takeoff and landing.

Drops by about one fifth ✓ — Right. With the other terms unchanged, lift scales with velocity squared, so 0.9 times the speed gives 0.9 squared, or 0.81, of the lift. That is roughly a 19 percent loss, not 10 percent. This squared relationship is one reason slow flight has so little margin: the last few knots can matter disproportionately.

Stays almost unchanged — No. A wing can look unchanged while its lift changes sharply because the moving air is part of the machine. Airspeed appears on the instrument panel, but it is also inside the physics: the dynamic pressure term contains velocity squared. Near the ground, 'almost the same speed' can be a very different lift state.