AQA GCSE Physics Foundation (8463), Foundation tier · Forces › Pressure in fluids
Practise Pressure in a fluid (p = F/A). 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A fluid (a liquid or a gas) exerts pressure on every surface it touches, and the force caused by the pressure acts at right angles to the surface. You need to recall and use p = F ÷ A, with pressure in pascals. This is in GCSE Physics only, not in Combined Science.
Key facts
Fluid = liquid or gas
Fluid pressure causes a force normal (at 90°) to any surface
p = F ÷ A (pressure = force normal to a surface ÷ area of that surface)
Units: p in Pa (1 Pa = 1 N/m2), F in N, A in m2
1 kPa = 1000 Pa
cm2 to m2: divide by 10 000
F = p × A gives the force on a surface
Notes
Fluids and pressure
diagram
A fluid is either a liquid or a gas.
The particles of a fluid collide with any surface in contact with it. The pressure in the fluid causes a force normal (at right angles) to that surface.
A fluid pushes on the bottom and the sides of its container, and on every face of an object placed in it.
The pressure in a fluid gives a force at right angles to every surface it touches.
Calculating pressure
pressure = force normal to a surface ÷ area of that surface: p = F ÷ A
p in pascals (Pa), F in newtons (N), A in square metres (m2).
1 Pa = 1 N/m2. Larger pressures are often given in kilopascals: 1 kPa = 1000 Pa.
Rearranged: F = p × A and A = F ÷ p.
Force, area and pressure
The force a fluid exerts on a surface is F = p × A, so at the same pressure a larger surface feels a larger force.
Example: water at a pressure of 150 000 Pa pushes on a 0.020 m2 porthole with a force of 150 000 × 0.020 = 3000 N.
The same force spread over a smaller area gives a greater pressure; over a larger area, a smaller pressure.
Area conversions
1 m = 100 cm, so 1 m2 = 100 × 100 = 10 000 cm2.
To convert cm2 to m2, divide by 10 000: 50 cm2 = 0.0050 m2.
1 m2 = 1 000 000 mm2, so divide mm2 by 1 000 000.
How to answer each type of question
Calculate pressure
2 marksGrade 5
Check the force is in N and the area in m2.
Write p = F ÷ A and substitute.
Give the answer in Pa.
Example. The water in a tank exerts a force of 7200 N on the base of the tank. The base has an area of 1.5 m2. Calculate the pressure on the base due to the water.
Show the model answerHide the model answer
p = 7200 ÷ 1.5 (1) p = 4800 Pa (1)
State what fluid pressure does at a surface
1 to 2 marksGrade 4
Say that the force from fluid pressure acts at right angles to the surface.
Say that it acts on every surface in contact with the fluid, not just downwards.
Example. A plastic bottle full of water has three small holes at the same depth, on different sides. Water comes out of all three holes, leaving each one at right angles to the side of the bottle. What does this show about the force caused by the pressure in a fluid?
Show the model answerHide the model answer
The pressure acts on every surface / in all directions, not just downwards (1). The force it causes acts at right angles (normal) to the surface (1).
Don’t lose marks
Dividing cm2 by 100 instead of 10 000.
Multiplying force by area when you should divide.
Mixing kPa and Pa, e.g. using kPa in the calculation but giving the answer in Pa.
Saying that fluid pressure only acts downwards. It acts on every surface, at right angles to it.
More tips
Memory tricks
Pressure is force ÷ area: the force goes on top.
Sense check: a smaller area should give a larger pressure.
kPa × m2 = kN. For example, 250 kPa × 0.012 m2 = 3 kN, a quick way to check your answer.
cm2 to m2: move the decimal point four places to the left.
Exam technique
p = F ÷ A must be recalled.
Show every unit conversion as a separate line: each can earn a method mark.
Give the unit Pa (or N/m2) with your answer.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that fluids are liquids or gasesBoth liquids and gases are fluids and both exert pressure.
Grade 4
State that fluid pressure acts normal to surfacesThe force caused by the pressure in a fluid acts at right angles to any surface.
Grade 5
Calculate pressure using p = F ÷ AFor example, 600 N on an area of 0.20 m2 gives 3000 Pa.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write down the equation that links area (A), force normal to a surface (F) and pressure (p).
\(p = \dfrac{F}{A}\)
Write down the equation that links area of a surface (A), force normal to the surface (F) and pressure (p).
\(p = \dfrac{F}{A}\)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Which two of these are fluids? Tick (✓) two boxes.[2]
Air
Ice
Steel
Water
(b) The pressure in a fluid causes a force on any surface in contact with the fluid. In which direction does this force act? Tick (✓) one box.[1]
At right angles (normal) to the surface
Parallel to the surface
Always vertically downwards
Always vertically upwards
(c) A force of 1200 N acts at right angles to a surface of area 0.30 m2. Calculate the pressure on the surface. Use the equation: \(\text{pressure} = \dfrac{\text{force normal to a surface}}{\text{area of that surface}}\)[2]
Show the answer and mark scheme
(a)Answer: Air; Water
(b)Answer: At right angles (normal) to the surface
(c)Answer: 4000 Pa
p = 1200 ÷ 0.30
4000 (Pa)
Question 2Medium7 marks
The water in a large aquarium exerts a pressure on a glass viewing window. The area of the window is 0.60 m2.
(a) Write down the equation that links area (A), force normal to a surface (F) and pressure (p).[1]
(b) The average pressure of the water on the window is 18 kPa. Calculate the force of the water on the window.[3]
(c) In which direction does the force of the water act on the window?[1]
(d) The aquarium is filled to a greater depth. Suggest what happens to the force of the water on the window. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: \(p = \dfrac{F}{A}\)
p = F ÷ A / pressure = force normal to a surface ÷ area of that surface
(b)Answer: 10 800 N
18 kPa = 18 000 Pa
F = 18 000 × 0.60
10 800 (N)
(c)Answer: At right angles to the glass (outwards).
at right angles to the window / normal to the glass (outwards)
(d)Answer: It increases, because the pressure of the water on the window increases.
the force increases
the (average) pressure (on the window) increases (because the water is deeper)