MDCAT Physics · Physics · Fluid dynamics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
These two equations describe an ideal fluid: one that cannot be compressed, has no viscosity and flows steadily. The equation of continuity is conservation of mass. The same volume must pass every cross-section each second, so the fluid moves faster where the pipe is narrower.
Bernoulli's equation is conservation of energy for the fluid. The pressure, the kinetic energy per unit volume and the potential energy per unit volume add up to the same total at every point along a streamline.
The result to remember is that, at the same level, the pressure is lower where the fluid moves faster. This explains the lift on an aeroplane wing, the carburettor, the spray gun and the venturi meter.
Students expect the pressure to be higher in the narrow part of a pipe because the fluid is squeezed. The fluid is faster there, so its pressure is lower.
Answer: 6.0 m s−1; 1.2 × 10−3 m3 s−1, which is 1.2 litres per second.
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