The Under Pressure simulation from PhET lets you explore pressure under and above water, and see how it changes when you change the fluid, the gravity, the shape of the container or the volume. You can discover that pressure depends on depth and density, not on how much liquid is around. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for pressure and fluids chapters in Tamil Nadu State Board (Samacheer Kalvi) Science and Physics.
What is the Under Pressure simulation?
This interactive simulation lets you study how pressure changes in air and in water. You can change the fluid, the strength of gravity, the shape of the container and the volume, and see what happens to the pressure at different depths. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
Pressure, depth, density and gravity
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to describe how pressure changes in air and water with depth, to describe which variables affect pressure, and to predict the pressure in a variety of situations.
Launch the Under Pressure simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Under Pressure simulation, step by step
- Predict first. Before you measure anything, say what you think will happen to the pressure as you go deeper.
- Measure at different depths. Check the pressure near the top and then deeper down, and notice how it changes.
- Change the fluid. Try a different liquid and compare the pressure at the same depth.
- Change the gravity. See how weaker or stronger gravity changes the pressure at the same depth.
- Change the container shape and volume. Check whether the pressure at a given depth changes when the container is wider, narrower or holds more liquid.
Tip for students: always compare readings at the same depth. That is how you find out which variable really matters.
The idea behind the simulation: P = P₀ + ρgh
P = P₀ + ρ × g × h
Here P is the total pressure in pascals (Pa), P₀ is the pressure of the air above the surface (about 101,325 Pa at sea level), ρ (rho) is the density of the fluid in kg/m³, g is the strength of gravity (about 9.8 m/s² on Earth) and h is the depth in metres. The term ρgh is the extra pressure caused by the fluid itself.
Worked example: At a depth of 10 m in fresh water (ρ = 1000 kg/m³), the extra pressure is 1000 × 9.8 × 10 = 98,000 Pa, which is 98 kPa. Adding the air pressure gives about 101,325 + 98,000 ≈ 199,000 Pa, or roughly 2 times the pressure at the surface.
Pressure key terms explained
These are the words you will meet while using the Under Pressure simulation, with simple meanings.
Pressure
The force pushing on each unit of area. It is force divided by area (P = F ÷ A).
Pascal (Pa)
The unit of pressure. One pascal is one newton per square metre.
Atmospheric pressure
The pressure of the air above us. It comes from the weight of the atmosphere.
Hydrostatic pressure
The pressure inside a fluid at rest. It increases with depth.
Depth (h)
How far below the surface of the fluid you measure. Measured in metres (m).
Density (ρ)
The mass of a material in each unit of volume, in kg/m³. A denser fluid gives more pressure at the same depth.
Gravity (g)
The pull that gives fluid its weight. Weaker gravity gives less pressure at the same depth.
Fluid
A liquid or a gas. Fluids flow and take the shape of their container.
Common mistakes: Thinking that pressure depends on the amount of liquid in the container instead of the depth, forgetting to add the atmospheric pressure when the total pressure is asked, and mixing up the density of a fluid with its weight.
Try the fluid pressure calculator (practice what you explore)
Choose a fluid, a depth and the strength of gravity to see the extra pressure from the fluid and the total pressure. Then tap a “what if” button.
What happens to pressure when you change a variable
| Change | Effect on pressure |
|---|---|
| Go deeper in the fluid | Pressure increases |
| Depth doubled | The extra pressure from the fluid doubles |
| Denser fluid (same depth) | Pressure increases |
| Weaker gravity (same depth) | Pressure decreases |
| Different container shape (same depth) | Pressure stays the same |
Classroom activities for teachers
The Under Pressure simulation fits a single 40-minute period.
- Predict and test (10 min): students write a prediction before each change, then check it in the simulation.
- Depth graph (10 min): students record the pressure at several depths and plot a graph of pressure against depth.
- Fluid comparison (10 min): compare the same depth in two fluids and explain the difference using density.
- Exit ticket (10 min): students explain in two sentences why a diver feels more pressure the deeper they go.
Looking for the matching chapters? Browse the Science and Physics books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the pressure and fluids lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of pressure
Score: 0 / 5
Under Pressure simulation: frequently asked questions
Is the Under Pressure simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
Why does pressure increase with depth?
The deeper you go, the more fluid is above you, and its weight presses down. That is why the pressure in a fluid grows in step with the depth, density and gravity.
Does the shape of the container change the pressure?
At the same depth in the same fluid, the pressure is the same whatever the shape of the container. Pressure depends on the depth, the density of the fluid and gravity.
The Under Pressure simulation turns an invisible quantity into something you can measure, compare and see. Explore it, take the quiz above, and keep experimenting until predicting the pressure feels easy.