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MENTAL MODEL · M5714

d'Alembert's Paradox

d'Alembert's Paradox
TechnicalHigh supportFluid Mechanics
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Version 1.0.0 · Updated 2026-07-30

CORE DEFINITION

In an ideal fluid (inviscid), a body moving at constant velocity experiences zero drag. This contradicts reality (even in air, there is drag). The reason is the neglect of 'viscosity', a small but crucial variable.

SCAFFOLDING EFFECT

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The fatal flaw of ideal models. Many perfect theoretical models (such as perfect competition markets, ideal fluids) fail in reality because they ignore some seemingly negligible 'friction' (such as transaction costs, viscosity).

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In an ideal inviscid fluid, a body moving at constant velocity experiences zero drag, but in reality, failure occurs due to 'viscosity', a small neglected quantity, revealing the blind spot of ideal models.

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Source support: Explicit

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    en.wikipedia.orghttps://en.wikipedia.org/wiki/D%27Alembert%27s_paradoxZH · Explicit
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