Drag Coefficient
Definition
A single number rating how aerodynamic a shape is — a sleek car scores ~0.25, a brick ~2.0.
Symbol: ·
Technical definition
C_d = 2F_d/(ρv²A), dimensionless; the drag force is F_d = ½ρv²C_dA.
Example
A cyclist in a racing tuck has C_dA ≈ 0.25 m² versus ~0.5 upright — halving aerodynamic drag at the same speed.
Common confusion
A low drag coefficient alone doesn't guarantee low drag — drag also scales with frontal area. A huge truck with a decent C_d still shoves enormous air aside.