Short note: air resistance

Air resistance is the force exerted by the air against the moving vehicle. It acts opposite to the motion and increases rapidly with speed.

Causes

1. Form (pressure) drag from the shape of the vehicle: high pressure in front and low pressure behind (about 55-60% of the total).
2. Skin friction of the air flowing over the body surface (about 10%).
3. Interference drag from the projections - mirrors, wheel wells, carriers, and gaps (the rest).

Expression

$$R_a = \tfrac12\,\rho\,C_d\,A\,v^2$$

where $\rho$ is the air density (1.225 kg/m³), $C_d$ the coefficient of drag (about 0.3-0.4 for cars, 0.6-1.0 for buses and trucks), $A$ the frontal area (m²) and $v$ the relative speed (m/s). The power needed to overcome it is $R_a v$, which is proportional to $v^3$.

Remarks

• Air resistance is small at low speed but becomes the largest resistance at high speed.
• It is reduced by streamlining the shape, covering loads and by reducing speed.
• In Nepal's hill roads at high altitude, the air is thinner, so the air resistance is lower.
• Headwind increases it and tailwind decreases it.