ForcesAQA GCSE Physics (8463)

Pressure and Pressure Differences in Fluids

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

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Worked examples5 Key formulae1 Definitions3 Common mistakes3

Step-by-step worked examples

01.Pool depth pressure
Foundation
Question
Calculate the pressure due to water at the bottom of a 2.5 m deep swimming pool (ρ = 1000 kg/m³, g = 9.8).
Solution
1. 1
  Apply formula.
  $p = h\rho g = 2.5 \times 1000 \times 9.8$
2. 2
  Calculate.
  $p = 24{,}500\,\text{Pa} = 24.5\,\text{kPa}$
Answer
24,500 Pa (24.5 kPa).
02.Submarine at depth
Higher
Question
A submarine is at 300 m depth in sea water (ρ = 1030 kg/m³). Calculate the water pressure on its hull (ignore atmospheric pressure).
Solution
1. 1
  Apply formula.
  $p = 300 \times 1030 \times 9.8 = 3.03 \times 10^6\,\text{Pa}$
2. 2
  Express.
  $p \approx 3\,\text{MPa} \approx 30\,\text{atm}$
Answer
~3.03 MPa (about 30 atmospheres). That's why submarine hulls are thick steel.
03.Calculate upthrust
Higher
Question
A submerged stone displaces 250 cm³ of water. Calculate the upthrust on it. ρ_water = 1000 kg/m³.
Solution
1. 1
  Convert volume.
  $V = 250\,\text{cm}^3 = 250 \times 10^{-6}\,\text{m}^3 = 2.5 \times 10^{-4}\,\text{m}^3$
2. 2
  Mass of water displaced.
  $m = \rho V = 1000 \times 2.5 \times 10^{-4} = 0.25\,\text{kg}$
3. 3
  Upthrust = weight.
  $U = mg = 0.25 \times 9.8 = 2.45\,\text{N}$
Answer
2.45 N upward.
04.Will it float?
Higher
Question
A block of cork has mass 0.30 kg and volume 1.2 × 10⁻³ m³. Will it float in water?
Solution
1. 1
  Density.
  $\rho = m/V = 0.30 / (1.2 \times 10^{-3}) = 250\,\text{kg/m}^3$
2. 2
  Compare.
  $250 < 1000\,\text{kg/m}^3 \Rightarrow \text{floats}$
Answer
Yes — cork density (250 kg/m³) is less than water (1000 kg/m³).
05.Why pressure falls with altitude
Foundation
Question
Explain why atmospheric pressure decreases as altitude increases.
Solution
1. 1
  Air column.
  $\text{At higher altitude, less air remains above you.}$
2. 2
  Weight.
  $\text{Smaller column of air means smaller weight pressing down.}$
3. 3
  Density.
  $\text{Air is less dense at altitude too — fewer molecule collisions per unit area.}$
4. 4
  Combined.
  $\text{Both effects reduce pressure.}$
Answer
Less air above means smaller column weight and lower air density → fewer molecule collisions on each surface → lower pressure.

Key formulae

Fluid pressure (depth)
$p = h \rho g$
- $p$
  — Pressure (Pa)
- $h$
  — Depth (m)
- $\rho$
  — Density of fluid (kg/m³)
- $g$
  — Grav. field strength (N/kg)
When to use
Pressure in a fluid at rest, due to the weight of fluid above. On AQA equation sheet.

Practice with flashcards

Card 1 of 30 known · 0 to review

Key definitions and keywords

UpthrustExaminer keyword: The upward force exerted by a fluid on a submerged or floating object. Equal to the weight of the fluid displaced.
Archimedes' principle: The upthrust on an object in a fluid equals the weight of the fluid the object displaces.
Atmospheric pressureExaminer keyword: The pressure due to the weight of air in the atmosphere; at sea level ≈ 100 kPa.

Common mistakes and misconceptions

Mistake
Thinking pressure depends on container shape or volume.
Why it happens
Intuition says wide = more water = more pressure.
How to avoid it
p depends only on h, ρ, g. A narrow tube of water at depth h has the SAME pressure at depth as a wide pool.
Mistake
Saying 'heavier objects sink'.
Why it happens
Weight more familiar than density.
How to avoid it
Compare DENSITY not weight. A massive aircraft carrier floats because its average density (with hollow interior) is less than water.
Mistake
Saying pressure decreases because gravity is weaker.
Why it happens
Confusing weight and gravitational field.
How to avoid it
Gravity (g) is almost the same at typical altitudes. Pressure falls because there's LESS AIR above, not weaker gravity.

ForcesAQA GCSE Physics (8463)

Pressure and Pressure Differences in Fluids

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

Previous Next

Master the topic

Worked examples5 Key formulae1 Definitions3 Common mistakes3

Step-by-step worked examples

01.Pool depth pressure
Foundation
Question
Calculate the pressure due to water at the bottom of a 2.5 m deep swimming pool (ρ = 1000 kg/m³, g = 9.8).
Solution
1. 1
  Apply formula.
  $p = h\rho g = 2.5 \times 1000 \times 9.8$
2. 2
  Calculate.
  $p = 24{,}500\,\text{Pa} = 24.5\,\text{kPa}$
Answer
24,500 Pa (24.5 kPa).
02.Submarine at depth
Higher
Question
A submarine is at 300 m depth in sea water (ρ = 1030 kg/m³). Calculate the water pressure on its hull (ignore atmospheric pressure).
Solution
1. 1
  Apply formula.
  $p = 300 \times 1030 \times 9.8 = 3.03 \times 10^6\,\text{Pa}$
2. 2
  Express.
  $p \approx 3\,\text{MPa} \approx 30\,\text{atm}$
Answer
~3.03 MPa (about 30 atmospheres). That's why submarine hulls are thick steel.
03.Calculate upthrust
Higher
Question
A submerged stone displaces 250 cm³ of water. Calculate the upthrust on it. ρ_water = 1000 kg/m³.
Solution
1. 1
  Convert volume.
  $V = 250\,\text{cm}^3 = 250 \times 10^{-6}\,\text{m}^3 = 2.5 \times 10^{-4}\,\text{m}^3$
2. 2
  Mass of water displaced.
  $m = \rho V = 1000 \times 2.5 \times 10^{-4} = 0.25\,\text{kg}$
3. 3
  Upthrust = weight.
  $U = mg = 0.25 \times 9.8 = 2.45\,\text{N}$
Answer
2.45 N upward.
04.Will it float?
Higher
Question
A block of cork has mass 0.30 kg and volume 1.2 × 10⁻³ m³. Will it float in water?
Solution
1. 1
  Density.
  $\rho = m/V = 0.30 / (1.2 \times 10^{-3}) = 250\,\text{kg/m}^3$
2. 2
  Compare.
  $250 < 1000\,\text{kg/m}^3 \Rightarrow \text{floats}$
Answer
Yes — cork density (250 kg/m³) is less than water (1000 kg/m³).
05.Why pressure falls with altitude
Foundation
Question
Explain why atmospheric pressure decreases as altitude increases.
Solution
1. 1
  Air column.
  $\text{At higher altitude, less air remains above you.}$
2. 2
  Weight.
  $\text{Smaller column of air means smaller weight pressing down.}$
3. 3
  Density.
  $\text{Air is less dense at altitude too — fewer molecule collisions per unit area.}$
4. 4
  Combined.
  $\text{Both effects reduce pressure.}$
Answer
Less air above means smaller column weight and lower air density → fewer molecule collisions on each surface → lower pressure.

Key formulae

Fluid pressure (depth)
$p = h \rho g$
- $p$
  — Pressure (Pa)
- $h$
  — Depth (m)
- $\rho$
  — Density of fluid (kg/m³)
- $g$
  — Grav. field strength (N/kg)
When to use
Pressure in a fluid at rest, due to the weight of fluid above. On AQA equation sheet.

Practice with flashcards

Card 1 of 30 known · 0 to review

Key definitions and keywords

UpthrustExaminer keyword: The upward force exerted by a fluid on a submerged or floating object. Equal to the weight of the fluid displaced.
Archimedes' principle: The upthrust on an object in a fluid equals the weight of the fluid the object displaces.
Atmospheric pressureExaminer keyword: The pressure due to the weight of air in the atmosphere; at sea level ≈ 100 kPa.

Common mistakes and misconceptions

Mistake
Thinking pressure depends on container shape or volume.
Why it happens
Intuition says wide = more water = more pressure.
How to avoid it
p depends only on h, ρ, g. A narrow tube of water at depth h has the SAME pressure at depth as a wide pool.
Mistake
Saying 'heavier objects sink'.
Why it happens
Weight more familiar than density.
How to avoid it
Compare DENSITY not weight. A massive aircraft carrier floats because its average density (with hollow interior) is less than water.
Mistake
Saying pressure decreases because gravity is weaker.
Why it happens
Confusing weight and gravitational field.
How to avoid it
Gravity (g) is almost the same at typical altitudes. Pressure falls because there's LESS AIR above, not weaker gravity.

Pressure and Pressure Differences in Fluids

Master the topic

Step-by-step worked examples

01.Pool depth pressure

02.Submarine at depth

03.Calculate upthrust

04.Will it float?

05.Why pressure falls with altitude

Key formulae

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions

Pressure and Pressure Differences in Fluids

Master the topic

Step-by-step worked examples

01.Pool depth pressure

02.Submarine at depth

03.Calculate upthrust

04.Will it float?

05.Why pressure falls with altitude

Key formulae

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions