Why Drag Increases with Speed

⚙️ Why Drag Increases with Speed

Drag is caused by the object colliding with air molecules. As the object moves faster, two things happen simultaneously:

1. More collisions per second — the object sweeps through more air in the same time
2. Each collision delivers more force — the relative speed of impact is higher

The combined effect means drag force is approximately proportional to speed squared (drag ∝ v²). This is why drag increases so rapidly once an object starts to move quickly — doubling your speed roughly quadruples the drag force you experience.

This also explains why streamlining matters so much at high speeds. At low speeds the drag is small anyway, but at motorway speeds a car experiences very significant drag. Aerodynamic shaping reduces the drag coefficient, lowering drag at any given speed and therefore raising the terminal velocity the car can reach.

What Affects Terminal Velocity?

• Surface area — larger area creates more drag at the same speed, so terminal velocity is lower. A parachute uses this principle.
• Shape (streamlining) — a streamlined shape creates less drag at the same speed, so terminal velocity is higher. Racing cars and aircraft are designed this way.
• Weight (mass) — a heavier object needs more drag to balance its weight, so it reaches a higher terminal velocity. A heavy skydiver falls faster than a light one.
• Fluid density — falling through denser air (at low altitude) creates more drag than falling through thin air (at high altitude), so terminal velocity is lower nearer the ground.