Air Resistance

Air resistance is the force that slows things down as they move through air. It is one form of friction, specifically the friction between a moving object and the gas around it. Air resistance is why a feather drifts gently to the ground while a stone drops fast, why parachutes work, why aeroplanes need streamlined shapes, and why cars are not box-shaped. Engineers spend years studying air resistance (often called drag) to design vehicles that move efficiently through the air.

  • What it doesSlows objects moving through airOpposite to motion
  • Also calledDragIn engineering terms
  • Stronger atHigher speedsRoughly speed squared
  • Stronger forLarger surface areaFacing the airflow
  • Terminal velocitySpeed where drag = weightFalling object stops accelerating
  • Human skydiver200 km/h spread eagle320 km/h head-first

Why air resists motion

Air is made of trillions of tiny gas molecules constantly moving in all directions. When you push your way through air, your body has to shove those molecules out of the way. The air molecules push back, slowing you down. The faster you go, the more molecules you hit each second, and the harder they push back.

What affects air resistance?

  • Speed: the faster you go, the bigger the resistance. Roughly, doubling your speed quadruples the air resistance (it grows as the square of speed).
  • Size and shape: bigger surface area facing the airflow means more drag. A flat plate has much more drag than a streamlined teardrop.
  • Air density: thicker air means more drag. High in the mountains, where air is thinner, planes can fly faster with less drag.
  • Shape (streamlining): smooth, rounded shapes let air flow around them with less turbulence. Boxy shapes leave swirls of air behind that create drag.

Terminal velocity

When something falls through the air, two forces act on it: gravity (pulling it down) and air resistance (pushing it up). Gravity stays constant, but air resistance grows as the object speeds up. Eventually they balance, and the object stops accelerating. The constant final speed is called terminal velocity.

Different objects have very different terminal velocities:

  • A feather: terminal velocity less than 1 m/s. Drifts down.
  • A raindrop: about 9 m/s.
  • A human skydiver (spread-eagled): about 55 m/s (200 km/h).
  • A human skydiver (head-first, arms tucked): about 90 m/s (320 km/h).
  • A 1 mm hailstone: about 5 m/s.
  • A 10 cm hailstone: about 60 m/s (210 km/h, dangerous).
  • A heavy metal ball: terminal velocity so high that for short falls (a few seconds) it never reaches terminal velocity.
Fact In a vacuum (no air), everything falls at the same rate, regardless of mass or shape. In 1971, astronaut David Scott famously demonstrated this on the airless Moon during Apollo 15. He dropped a hammer and a feather at the same time. They hit the lunar surface together, 1.3 seconds later. On Earth, the feather would have drifted slowly down because of air resistance.

Parachutes

A parachute works by drastically increasing the surface area of a falling person, so air resistance becomes much larger. A skydiver with their parachute open has roughly 60 square metres of canopy facing the airflow, compared with about 0.7 square metres in normal freefall. The huge area lowers terminal velocity from around 200 km/h (deadly) to about 18 km/h (a safe landing speed).

Skydivers usually open their parachutes around 600 metres above the ground, giving the chute time to fully deploy and the skydiver time to steer toward the landing zone.

Streamlining

Vehicles designed to go fast through air or water are streamlined: shaped to let the fluid flow smoothly around them. The classic streamlined shape is a teardrop: rounded at the front, tapering at the back.

Why a teardrop? At the front, the rounded shape gently pushes air aside without smashing into it. At the back, the long taper lets the air close in smoothly behind the object, instead of creating turbulent swirls that drag it backwards.

You can see this shape in:

  • Aircraft fuselages.
  • Submarine hulls.
  • Bullet trains.
  • Modern sports cars.
  • Dolphins, sharks and tuna.
  • Birds of fast flight, like peregrine falcons.
Did you know? A modern Formula 1 car generates so much air resistance at top speed that if all its drag became downward force, it could drive upside-down on the ceiling of a tunnel. F1 designers carefully shape the wings, body and floor of the cars to create huge amounts of downforce, pushing the car into the track for better cornering. The trade-off is huge drag, which slows top speed.

Drag on aircraft

For aircraft, drag is the enemy of efficiency. Every kilo of fuel a plane carries to push through the air is a kilo less of cargo, passengers or longer-range fuel. Reducing drag has been a major focus of aircraft design for decades.

Modern airliners have:

  • Carefully designed wing tips (winglets) that reduce the drag from swirling wingtip vortices.
  • Riblets and other tiny surface textures that subtly reduce drag.
  • Smooth, well-blended joins between wings and fuselage.
  • Retractable landing gear (because wheels in the airflow create huge drag).
  • Wing flaps that extend to give extra lift at slow speeds (take-off and landing) and retract to reduce drag at cruise.
Try this Cut a piece of paper into a small flat square. Crumple another paper into a tight ball. Drop both from the same height at the same time. The crumpled ball reaches the ground much faster. They have the same weight, so why? Because the flat square has much more surface area facing the airflow, so much more air resistance. The crumpled ball is essentially a tiny streamlined shape with little drag. Different shapes, same mass, very different falling speeds.
Deeper dive: how peregrine falcons hit 320 km/h in a dive

The peregrine falcon is the fastest animal on Earth. In its hunting dive (called a "stoop"), it can reach speeds of over 320 km/h: faster than any other living creature. The bird dives from high altitude onto its prey (usually other birds in flight), striking them with a clenched foot at terrifying speed.

The peregrine has evolved remarkable adaptations to manage the air resistance at such speeds:

  • Streamlined body: when the falcon tucks its wings and dives, its body becomes a sleek teardrop with minimum drag.
  • Stiff, narrow wings: held tight against the body during the stoop, they create much less drag than during normal flapping flight.
  • Tiny nostril cones: the falcons nostrils have small bony cones inside them that break up the airflow, stopping the rushing wind from forcing too much air into the lungs. Fighter pilots and supersonic aircraft engineers have studied these natural inlet cones for design ideas.
  • Special protective eyelids (called nictitating membranes) that close to protect the eyes from wind and debris during the dive.
  • Reinforced skull and dense muscles to survive the impact at the end of the stoop, when the falcon hits its prey at 80 km/h or more (after slowing down for the strike).

The peregrine has been used by falconers for over 4,000 years. Today they are also used at airports to scare off other birds and prevent dangerous bird strikes on jet aircraft. Once driven nearly extinct by the pesticide DDT in the 1960s, populations have recovered dramatically. Peregrines now nest in many British cities, including London (where pairs live on tall buildings like Tate Modern, hunting pigeons in the streets below). All thanks to mastery of air resistance.

For more, see friction and what is a force.