Air feels like nothing at all. Wave your hand through it slowly and you barely notice it is even there. But it is there — the room is packed with air, from the floor to the ceiling, filling every gap. And the moment you try to move through it quickly, you can suddenly feel it.
Stick your hand flat out of a car window on the motorway. Whoosh — the air slams against your palm and shoves it backwards. That invisible shove has a name: drag (also called air resistance). To move forwards you have to barge all that air out of the way, and it pushes right back on you. Water does exactly the same thing, only far harder.
Air and water are both made of countless tiny bits, too small to see. When you move, you can't magically pass through them — you have to shove them aside. And whenever you push on something, it pushes back on you. So the harder you have to shove the air or water out of your path, the harder it shoves back. That back-push, always aimed against your motion, is drag.
Three things make drag grow, and every one of them matters:
Try this in a pool, or even a bathtub. Hold your hand flat, like a paddle, and sweep it through the water. It's hard work — the water piles up against that wide flat surface and drags on it heavily. Now curl your hand into a slim fist and push it through the same way. It slides through easily. Same hand, same water, same speed — but the flat, wide shape catches far more, so it feels far more drag.
This is the biggest secret about drag: the size and shape of the surface facing the way you're going is what counts. A wide sail of a shape catches the air like a net; a narrow, pointy shape barely disturbs it. That is why an open umbrella tries to fly out of your hand in a gale, but a closed one does not.
Ride a bike gently down the street and the air is barely a whisper on your face. Now pedal flat-out. Suddenly the air is roaring past your ears and pressing hard against your chest, trying to hold you back. You didn't change shape — you just went faster, and drag grew with your speed.
It's the same for a car. Trundling along a quiet lane, air resistance hardly matters. But out on the motorway, the car has to shove aside a huge wall of air every second, and drag becomes the main thing slowing it down. That's why a fast car is built low and smooth — to give the rushing air as little as possible to push against.
The peregrine falcon is the fastest animal on the whole planet. When it spots a pigeon far below, it folds its wings tight against its body, pulls its feet in, and turns itself into a smooth, pointed dart. In this streamlined dive — called a stoop — it can plunge at over 300 kilometres an hour, faster than a racing car.
If it dived with its wings spread wide, the air would catch them like two big parachutes and slow it right down. By tucking in tight, it gives the air almost nothing to push against, and drops like a thunderbolt. Every fast animal, car and plane uses the same trick: a smooth, slim, streamlined shape that lets the air slide past.
Usually drag is a nuisance — it slows the cyclist and wastes the car's fuel. But sometimes drag is exactly what saves your life. A person jumping from a plane has their weight pulling them straight down, faster and faster. That would end very badly. So they open a parachute: an enormous, wide canopy that catches a colossal amount of air.
Because the canopy is so big and flat, the air pushes up on it enormously hard — so much that the up-push nearly matches the person's weight, and the terrifying fall becomes a gentle, floating drift down to the ground. Drag alone turns a deadly plummet into a safe landing.
Drag the slider to change the size of the parachute. A tiny canopy catches barely any air, so the up-push is small and the weight wins — down you plummet. Make the canopy big and the air push (the up-arrows) grows, holding you up so you drift down gently.
Air is thin and light, so unless you're moving fast its push is small. Water is a different story. It is far thicker and heavier, so it pushes back much, much more than air does. Try to run across a swimming pool and you can barely stagger forwards — the water drags on your legs like thick treacle, even when you're moving quite slowly.
That's why a fish's body is so beautifully smooth and pointed, and why a boat is shaped to slice cleanly through the waves. In water, where drag is so fierce, being streamlined isn't a nice extra — it's the only way to move at all without exhausting yourself.
At the Olympics, races are won and lost by a hundredth of a second — so swimmers do everything they can to cut their drag. Many shave off every hair on their arms, legs and even their heads, so the water flows past their skin as smoothly as possible.
They also wear special skin-tight suits that hug the body with no baggy, flapping folds for the water to grab. A loose T-shirt would catch the water like a sail and drag them back; a smooth, streamlined swimmer slips through it like a fish. When water pushes back this hard, even a tiny bit less drag can win the gold.