Lift and drag vary directly with the density of the air.

• A. Velocity
• B. Pressure
• C. Air Density
• D. Temperature

Answer: (C)

Lift and drag scale with dynamic pressure, which includes the air density. The dynamic pressure is q = 0.5 * rho * V^2, and both lift and drag are proportional to q (L = CL q S and D = CD q S). So, keeping velocity and the wing shape fixed, increasing air density makes q larger, causing lift and drag to rise in direct proportion to density. If you double the density at the same speed, lift and drag roughly double.

As for the other factors, velocity also affects lift and drag through the V^2 term in dynamic pressure, so changing speed changes both quantities, but that isn’t a direct, sole dependence on density. Pressure differences arise from how air flows around the wing and from velocity, and temperature affects density via the ideal gas law. The direct link being tested is that air density itself causes lift and drag to scale in direct proportion.