The floating ball demonstration shows how a ball can be suspended in mid-air by a stream of moving air — a delightful real-world application of the Bernoulli effect combined with entrainment. When a ball is placed in an upward-directed stream of air (from a hair dryer or leaf blower), it hovers stably rather than being blown away sideways. Children are amazed by what appears to be magic, but the physics is straightforward and elegant.
The ball floats because of two interacting effects. First, the upward momentum of the air jet supports the ball's weight — the air exerts an upward force. Second, if the ball drifts sideways, it experiences a pressure difference: the air flowing around the nearer side of the ball speeds up (lower pressure, Bernoulli), while the farther side has less air flow and higher pressure. This net inward pressure force pushes the ball back to the centre of the jet, creating a stable equilibrium.
This principle appears in numerous real applications. Aircraft wings generate lift by accelerating air over the curved upper surface (lower pressure) relative to the flatter lower surface (higher pressure). Carburettors use the Venturi effect to mix fuel with air. Industrial vacuum systems use air jets (Venturi ejectors) to create suction without moving parts. The Coanda effect — the tendency of a fluid jet to cling to an adjacent surface — is related and is exploited in aircraft flap design and fluidic devices.
When the ball drifts toward the edge of the jet, the faster-moving air on the inside of the ball creates lower pressure (Bernoulli effect), while the slower air outside creates higher pressure. This pressure difference pushes the ball back toward the centre, making the position stable. The air stream acts like an invisible elastic tether.
Bernoulli's principle states that in a flowing fluid, faster-moving fluid has lower pressure. It follows from conservation of energy: as fluid speeds up (higher kinetic energy), its pressure (potential energy per unit volume) must decrease. This pressure difference between fast and slow regions of airflow creates lift on wings, suction in vacuum pumps, and the ball-suspension phenomenon.
Yes! Hold a table tennis ball or light plastic ball in the stream of an upward-pointing hair dryer or small fan. Gently release the ball — it should hover stably in the air stream. Try tilting the dryer slightly; the ball follows the jet. Use a heavier ball and see how much air speed is needed. This is a classic physics demonstration suitable for all ages.
The Coanda effect is the tendency of a moving fluid to attach itself to and follow along an adjacent curved surface. When an air jet curves around the ball's surface, the entrainment of surrounding air and the jet's own curvature create the conditions for the ball to be pulled into the jet. Henri Coanda discovered this effect in 1910 in the context of jet propulsion.
Both involve the Bernoulli effect and flow attachment. An aircraft wing (aerofoil) is shaped so that air must travel faster over the curved upper surface than the flatter lower surface, creating lower pressure above and higher pressure below — net upward lift. The ball in the air stream similarly experiences asymmetric pressure when off-centre, restoring it to the centre, just as an aircraft maintains altitude by balancing lift against weight.