Fluids
Venturi tube
Watch flow speed up in the narrow throat — pressure drops (Bernoulli).
Explore the Venturi effect and Bernoulli's principle in a constricted pipe flow. Watch pressure drop and speed increase where the tube narrows.
Bernoulli effect
In a Venturi tube, fluid speeds up in a narrow throat and pressure drops.
Conservation of mass requires faster flow where the cross-section is smaller. Bernoulli's equation links higher speed to lower pressure in the throat.
- A₁v₁ = A₂v₂
- P + ½ρv² = constant
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Water flows through a pipe that narrows partway through, like a garden hose with your thumb over the end. What happens to the water's pressure in the narrow section?
Predictions to weigh
- Pressure increases in the narrow section because the water is more compressed.
- Pressure decreases in the narrow section because the water speeds up.
- Pressure stays the same throughout — only speed changes.
Variable roles
What you set:
- Wide-section area A₁ (m²)
- Throat area A₂ (m²)
- Inlet speed v₁ (m/s)
What you measure:
- Throat speed v₂ (m/s)
- Pressure change ΔP = P₂ − P₁ (kPa)
How the investigation runs
- Open the fluid-venturi preset and press Reset. Water flows steadily through a pipe with a wide section (A₁) and a narrow throat (A₂).
- Enable the throat-speed and pressure-drop readouts.
- Set the wide-section area, throat area, and inlet speed for each trial, and record the throat speed and pressure-drop readings.
Governing equation
Continuity Equation — A₁·v₁ = A₂·v₂
For an incompressible fluid in a pipe, the flow rate (area × speed) must be the same everywhere along the pipe: A₁v₁ = A₂v₂. Narrowing the pipe forces the fluid to speed up.
Bernoulli's Equation — P₁ + ½ρv₁² = P₂ + ½ρv₂²
Along a horizontal streamline, a fluid's pressure plus its kinetic-energy-per-volume stays constant: P₁ + ½ρv₁² = P₂ + ½ρv₂². Where speed increases, pressure must decrease to compensate.
What the printable worksheet asks students to work out
- For one trial, compute v₂ = A₁v₁/A₂ from continuity, then ΔP = P₂ − P₁ = ½ρ(v₁² − v₂²) from Bernoulli's equation. Compare both to the readings — ΔP comes out negative, which is what "the pressure fell at the throat" looks like as a signed number.
- Explain, in terms of energy conservation, why a fluid's pressure must drop where its speed increases in a horizontal pipe.
Where this shows up beyond the lab
- Carburetors and perfume atomizers use the Venturi effect to draw in a second fluid at the narrow throat. Using your pressure-change data, explain why a low-pressure region there can pull liquid upward through a side tube.
- The fluid-wing experiment uses this same continuity + Bernoulli model with air instead of water. Would you expect the pressure difference to be larger or smaller for the same speeds and areas? Why?
- AP Physics 1 — Unit 8: Fluids
- General High School Physics — Fluids & pressure
- NGSS High School Physics — Forces and Newton's second law
- Welcome to Fluid Venturi
- Press Play
- Fast throat, low pressure
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.