Why does the balloon hover instead of falling?

• A hair dryer blasts a column of air upward. That air is moving fast. And fast‑moving air has lower pressure than the still air around it. Daniel Bernoulli discovered this in the 1700s: when a fluid speeds up, its pressure drops.

Think of it like a busy motorway. Cars travelling at high speed stay in their lanes — they don’t push sideways. But the still air beside the motorway? It presses in from all directions with full force. The balloon sits right in this situation:

• Low pressure in the fast airstream

• Normal pressure from the surrounding still air

That surrounding air squeezes inward, holding the balloon inside the rising column. Gravity pulls down, but the pressure difference pushes up harder. The balloon hovers.

Why the coin matters?

• A balloon on its own is too light and too symmetrical. It spins, wobbles, and drifts — it has no “down”. Drop a coin inside and everything changes. The coin settles at the bottom, giving the balloon a heavy end and a light end. Now it behaves like a ship with ballast:

• the heavy end stays down

• the balloon stabilises

• the wobbling stops

The coin doesn’t help the balloon float — it helps it stay upright long enough for the air column to catch and hold it.

Why does the balloon follow the airstream when you tilt the dryer?

• This is the Coanda Effect — the tendency of moving air to cling to curved surfaces. When you tilt the hair dryer, the airstream tilts too. Instead of falling out of the stream, the balloon “chases” it.

Here’s what’s happening:

• The curved surface of the balloon bends the fast air around it.

• The air sticks to the balloon’s surface and curves away.

• Where the air curves away, the pressure drops.

• Higher pressure on the opposite side pushes the balloon back in.

The balloon constantly self‑corrects — always nudged back toward the centre of the airstream. It behaves like a satellite hugging its orbit: drifting slightly, correcting instantly, never escaping.

Where have you seen this before?

The same two principles — Bernoulli and Coanda — work together in:

• every aircraft wing

• every helicopter rotor

• every hovercraft

• every leaf that lifts and flutters above a campfire

Your hair dryer just demonstrated the physics of flight using nothing more than a balloon and a coin.

Try next

To see the same self-correcting Coanda behaviour on a smaller scale — and without a hair dryer — try [The Invisible Cushion], where a handmade card cone and a single breath do the same job. And if you want to see Bernoulli working sideways instead of vertically, [The Stubborn Balloons] shows what happens when fast air runs between two objects instead of under one.