A Kitchen Sink Experiment that Challenges the Use of Bernoulli to Explain Lift
How an aeroplane wing generates lift.
Introduction:
Many textbooks explain lift in terms of a “pressure difference” created above and below a wing, invoking Bernoulli’s principle. While pressure certainly plays a role in fluid mechanics, this common explanation can be misleading. In fact, a simple kitchen experiment demonstrates that lateral flow deflection can occur without any continuous surface producing a pressure differential. The real driver? Momentum exchange. Which of course relies on the discoveries made by Isaac Newton.
The Experiment:
Take a standard mesh strainer — the kind used to smooth sauces. Hold it under a running tap so that the water hits the convex side of the mesh. Observe:
1. When the stream hits the flat, central portion of the mesh nearly at right angles, the water passes through the mesh with minor distortion. The flow continues mostly straight down.
2. When the curved sides of the mesh are presented to the water stream, the flow bends outward, following the curvature of the surface.
This bending is the Coandă effect in action. The key point: the mesh has gaps. There is no continuous surface to create a classic Bernoulli pressure differential. The deflection occurs because the mesh changes the local flow of the water, altering the momentum of the stream. The smaller the angle of attack, ie the higher up the mess you move the stream of water from the tap the more pronounced the bending.
Implications:
* This simple observation challenges the textbook emphasis on pressure as the cause of lateral flow in certain cases.
* The effect is robust: it works whether the flow is laminar or turbulent.
* In aerodynamics, wings and propellers are similarly redirecting momentum. The pressure field emerges as a consequence, not the primary cause.
Conclusion:
Fluid behavior still contains surprising subtleties, even in everyday settings. The kitchen strainer experiment is a vivid, accessible demonstration that momentum exchange is the real driver behind flow deflection — and that relying solely on a Bernoulli-based explanation can obscure deeper understanding.
NB. This is a summary of a dialogue with ChatGPT v5 which summarised a long prompt for me.