---
title: "Short note: air resistance"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:air-resistance-note"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/air-resistance-note
site: BCT Engineering
---

# Short note: air resistance

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2065 Kartik (old course), 4 marks
- Video: [Understanding Aerodynamic Drag | How Air Resistance Affects Vehicles](https://www.youtube.com/watch?v=yeFYCBXAD3k) — Innovation Discoveries
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/air-resistance-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Write a short note on air resistance.

## Short answer

Air resistance opposes motion from form drag, skin friction, and interference drag. It is given by ½ ρ Cd A v², rises with speed squared, and dominates at high speeds; it can be reduced by streamlining and lower speeds.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which expression gives air resistance?
   A. R_a = rho Cd A v
   B. R_a = 0.5 rho Cd A v^2
   C. R_a = 0.5 rho Cd A v^3
   D. R_a = f W
   Answer: B. Air resistance = 0.5 x density x drag coefficient x frontal area x v^2. (Key point: Air resistance expression)

2. The power needed to overcome air resistance is proportional to
   A. v
   B. v^2
   C. v^3
   D. 1/v
   Answer: C. Power = R_a x v, and R_a is proportional to v^2, so power varies as v^3. (Key point: Power to overcome air resistance)

3. Which component is the largest contributor to air resistance?
   A. Form (pressure) drag from vehicle shape
   B. Skin friction
   C. Interference drag from projections
   D. Tyre deformation
   Answer: A. Form drag from high pressure in front and low pressure behind accounts for about 55-60% of the total. (Key point: Main cause of air drag)

4. Which statement about air resistance is correct?
   A. It is largest at low speed
   B. It is higher at high altitude because air is thinner
   C. It is the largest resistance at high speed and lower where the air is thinner
   D. A tailwind increases it
   Answer: C. It grows rapidly with speed and depends on air density, which is lower at high altitude; a tailwind reduces it. (Key point: Effect of altitude and speed)

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