Pressure in fluids: GCSE Physics
Pressure in liquids and gases, floating, atmospheric pressure and hydraulics
The short version
p = F/A and p = hρg in a column of liquid. Atmospheric pressure falls with height. Hydraulics multiply force.
Key idea
P in Pa (N/m^2); F in N; A in m^2. Same F, smaller A -> larger P
Grade 2–3: A sharp point has a tiny area so the push is concentrated.
Grade 5–6: I can use P = F/A and change cm^2 to m^2.
Grade 9: I convert units without dumping and I can use the perpendicular component of force.
Go a bit deeper
h is vertical depth in m; ρ in kg/m^3; g ≈ 9.8 N/kg. Not the length down a slope
Grade 2–3: Deeper water means more water above you, so more squash.
Grade 5–6: I can substitute into P = hρg with h in metres.
Grade 9: I use vertical h, I can add atmospheric pressure, and I know a wider lake does not increase P at the same depth.
Worked example
Worked example
Grade 2–3: A sharp point has a tiny area so the push is concentrated. Grade 5–6: I can use P = F/A and change cm^2 to m^2. Grade 9: I convert units without dumping and I can use the perpendicular component of force.
Lessons in this topic
- Pressure and solids
- Pressure in a liquid column
- Upthrust and floating
- Atmospheric pressure
- Hydraulics
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Use p = F/A and p = hρg, explain floating using upthrust, and calculate hydraulic forces with equal pressure.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
Pressure p =
1 pascal equals
Liquid pressure at depth h is
At the same depth in the same liquid, pressure is
Common mistakes
- Do not leave area in cm^2. 1 m^2 = 10 000 cm^2. Pressure is not a force - do not give the answer in newtons.
- Do not use the slanted length of a slide as h. Do not use g = 9.8 m/s^2 in the wrong unit story - here it is N/kg, same number.
- Boats do not float "because air is lighter than steel" as a complete answer - you need displaced water weight. Upthrust is not the same as atmospheric pressure.
- There is still plenty of air on a mountain - pressure is lower, not zero. A vacuum cleaner does not "suck" with a magic pull force; outside air pushes.
Answers and working
Frequently asked questions
Is GCSE Pressure in fluids on Combined Science and Triple?
Pressure in fluids is Triple-only Physics. Combined Science students can skip it.
Which exam board is this Pressure in fluids page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Pressure in fluids Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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