Pressure in Fluids
Get to grips with Pressure in Fluids for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: pressure in a liquid increases with depth and density, calculated using p = hρg, and produces an upthrust on submerged objects.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A fluid is either a liquid or a gas. The pressure in a fluid causes a force to act normally, meaning at right angles, to any surface. This is a Triple-only topic.
The equation for pressure is pressure = force normal to a surface ÷ area of that surface, measured in pascals. A pascal is one newton per square metre.
The pressure at a depth in a liquid is given by pressure = height of column × density × gravitational field strength. Pressure increases with depth because there is a greater weight of liquid above, and it increases with density for the same reason.
Revision notes
Pressure and area
Pressure = force normal to a surface ÷ area of that surface, p = F ÷ A.
Force in newtons, area in square metres, pressure in pascals. One pascal is one newton per square metre. The force acts at right angles to the surface.
Pressure in a column of liquid
Pressure = height of column × density of liquid × gravitational field strength, p = hρg.
Height in metres, density in kg/m³, g in N/kg, pressure in pascals. Note this gives the pressure due to the liquid alone.
Why pressure increases with depth
The deeper you go, the greater the weight of liquid above that point.
That weight acts over the same area, so the pressure is greater. A denser liquid gives a greater pressure at the same depth, because the column above weighs more.
Key points
- A fluid is a liquid or a gas.
- Pressure = force ÷ area.
- Pressure is measured in pascals.
- The force acts normally to the surface.
- Pressure in a liquid = height × density × g.
- Pressure increases with depth and density.
Worked examples
Example 1
A force of 200 N acts normally on an area of 4 m². Calculate the pressure. [2 marks]
Model answer
Pressure = force ÷ area = 200 ÷ 4 (1 mark) = 50 Pa (1 mark).
Example 2
Calculate the pressure at a depth of 3 m in water. Density of water = 1000 kg/m³, g = 9.8 N/kg. [3 marks]
Model answer
Pressure = height × density × g = 3 × 1000 × 9.8 (1 mark). 3 × 1000 = 3000 (1 mark). 3000 × 9.8 = 29 400 Pa (1 mark).
Example 3
Explain why pressure in a liquid increases with depth. [2 marks]
Model answer
There is a greater weight of liquid above a point that is deeper down (1 mark), and this weight acts over the same area, so the pressure is greater (1 mark).
Common mistakes
Using a force that is not normal to the surface.
Only the component at right angles to the surface counts.
Forgetting the density in the liquid pressure equation.
All three quantities are needed: height, density and g.
Giving the unit as newtons.
Pressure is measured in pascals, or N/m².
Saying pressure depends on the volume of liquid.
It depends on the depth and density, not the total volume.
Exam tips
- Use the perpendicular force in the pressure equation.
- State pascals as the unit.
- Explain depth dependence through the weight of liquid above.
- Remember this is a Triple-only topic.
Key terms
- Fluid
- A liquid or a gas.
- Pressure
- Force per unit area, in pascals.
- Normal
- At right angles to a surface.
- Density
- Mass per unit volume, in kg/m³.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.