AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Pressure in a fluid 1: p = F / A
This equation defines pressure as the normal force divided by the area over which it acts.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation defines pressure as the normal force divided by the area over which it acts. In symbols, p is pressure in pascals, F is the normal force in newtons and A is the area in square metres. A force of 50 N spread over 0.25 m² gives p = 50 ÷ 0.25 = 200 Pa. The equation can be rearranged to F = p × A or A = F ÷ p. Because area appears on the bottom, a smaller area gives a larger pressure for the same force, which is why a drawing pin has a sharp point. When using the equation, convert areas such as 20 cm² to 0.0020 m² before substituting, and make sure the force is perpendicular to the surface.
What earns the marks
- Identify p as pressure in pascals, F as normal force in newtons and A as area in square metres.
- Substitute values into p = F / A and evaluate correctly.
- Rearrange the equation to calculate force as F = p × A or area as A = F / p.
- Convert areas from cm² or mm² to m² before substitution.
- Interpret a large pressure as a large force acting on a small area.
Common mistakes and how to put them right
Substituting an area in cm² directly into the equation.
Put it right: Convert to m² first, for example 20 cm² = 0.0020 m².
Dividing area by force instead of force by area.
Put it right: Keep F on top and A on the bottom.
Using a slanted force as F without resolving it.
Put it right: Use only the component normal to the surface.
Exam tips
- Quote the equation, show the substitution, then state the answer with the unit Pa.
- If the answer looks very large or very small, check the area conversion before moving on.
- Use standard symbols p, F and A in working so the examiner can follow your method.
Try the lesson
The start of the Pressure in a fluid 1: p = F / A lesson, made from checked teaching notes for this specification point.
Have a go first
A force of 50 N acts normally on a surface of area 2.0 m². Use p = F / A to calculate the pressure.
Listen
This equation defines pressure as the normal force divided by the area over which it acts. In symbols, p is pressure in pascals, F is the normal force in newtons and A is the area in square metres. A force of 50 N spread over 0.25 m² gives p = 50 ÷ 0.25 = 200 Pa. The equation can be rearranged to F = p × A or A = F ÷ p. Because area appears on the bottom, a smaller area gives a larger pressure for the same force, which is why a drawing pin has a sharp point. When using the equation, convert areas such as 20 cm² to 0.0020 m² before substituting, and make sure the force is perpendicular to the surface. A strong answer earns marks for these points. Identify p as pressure in pascals, F as normal force in newtons and A as area in square metres. Substitute values into p = F / A and evaluate correctly. Rearrange the equation to calculate force as F = p × A or area as A = F / p. Convert areas from cm² or mm² to m² before substitution. A common slip: Substituting an area in cm² directly into the equation. Instead, convert to m² first, for example 20 cm² = 0.0020 m². Exam tip: Quote the equation, show the substitution, then state the answer with the unit Pa.
See it
Key words
Say what each word means before you turn the card.
Pressure
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Rearranging equations
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Unit conversion
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Pressure and area
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See every definition
- Pressure
- The normal force divided by the area over which it acts, measured in pascals.
- Rearranging equations
- The equation can be rearranged to F = p × A or A = F ÷ p.
- Unit conversion
- Areas such as 20 cm² must be converted to 0.0020 m² before substituting into the equation.
- Pressure and area
- Because area appears on the bottom, a smaller area gives a larger pressure for the same force.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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