Pascal’s Vases – Different Shapes, Same Pressure?
One of the most surprising demonstrations in physics is discovering that the shape of a container does not determine the pressure at the bottom.
At first glance, students are convinced it must.
A tall thin vase looks as though it should exert less pressure than a wide heavy-looking one.
Surely the container holding more water must produce the greatest pressure?
That is exactly why Pascal’s Vases are such a useful teaching demonstration.
What Are Pascal’s Vases?
Pascal’s Vases are a set of differently shaped containers connected to the same base.
Typically:
- One is tall and narrow
- One widens outwards
- One narrows towards the top
- Another may have curved sides
Despite their different shapes and water volumes, when the water level is the same height, the pressure at the base is also the same.
That idea feels completely wrong to many students at first.
And that is what makes the experiment memorable.
The Key Physics Idea
Hydrostatic pressure depends on:
- The density of the liquid
- Gravity
- The depth of the liquid
Not the total volume.
The equation is:
Where:
- = pressure
- = density
- = gravitational field strength
- = depth of liquid
The width or shape of the container does not appear in the equation at all.
That is the important conceptual breakthrough.
Experiment 1 – Comparing Water Pressure
The simplest experiment is to fill each vase to exactly the same height.
Students predict which vase will create the largest pressure at the bottom.
Most choose the widest one because it contains the most water.
Using:
- A pressure sensor
- A manometer
- Or even observing water jets from holes at the base
students quickly discover the pressure is identical.
This creates one of those excellent “That can’t be right…” moments in the lab.
And those moments are where real learning starts.
Experiment 2 – Water Jets
If each vase has a small hole at the same depth, the water jets travel roughly the same horizontal distance.
Again, students expect the larger vase to produce a more powerful stream.
But the speed of the water leaving depends mainly on the pressure due to depth.
Equal depth → equal pressure → similar jet behaviour.
A wonderfully visual demonstration.
Experiment 3 – Measuring Force on the Base
Different shaped vessels can contain different masses of water while producing the same pressure at the base.
Students naturally assume:
“More water means more downward force.”
But some of the weight is supported by the sides of the container depending on the shape.
This opens discussions about:
- Resultant forces
- Vector components
- Pressure vs total force
- Engineering design
It is an excellent bridge between GCSE and A-Level thinking.
Experiment 4 – PASCO Pressure Sensors
Using modern PASCO pressure sensors connected to software such as PASCO Capstone transforms the experiment.
Students can:
- Record live pressure readings
- Compare multiple vessels
- Plot pressure against depth
- Investigate linear relationships
The graph obtained is typically a straight line:
Students can then experimentally verify the hydrostatic pressure equation rather than simply memorising it.
That is a far deeper level of understanding.
Common Student Misconceptions
“More water means more pressure.”
Not necessarily.
Pressure depends on depth, not volume.
“Wider containers push harder.”
Only if the depth changes.
“Pressure and force are the same thing.”
They are closely related but not identical.
Pressure is force per unit area.
Pascal’s Vases help students separate these ideas clearly.
Why Demonstrations Matter
Students often understand equations only superficially.
But seeing identical pressure readings from wildly different vessels creates a mental conflict that forces genuine understanding.
That is why practical demonstrations remain so important in physics teaching.
A surprising experiment is remembered far longer than a copied note.
And Pascal’s Vases are full of surprises.
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