AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Pressure in a fluid 1: force, F, in newtons, N
Force, F, is a push or pull measured in newtons, N, and in pressure calculations it is the force acting perpendicular to a surface.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 7 of 9
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Force, F, is a push or pull measured in newtons, N, and in pressure calculations it is the force acting perpendicular to a surface. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s². In p = F ÷ A, F is the total force normal to the area, so a 600 N person standing on both feet exerts 600 N on the floor whatever the area. If the area changes, the force stays the same but the pressure changes. To find force from pressure, rearrange to F = p × A, keeping area in m² so the answer is in N. Weight is a force and is calculated using W = m × g, where g is about 9.8 N/kg on Earth. Always identify which force acts perpendicular to the surface before substituting into the pressure equation.
What earns the marks
- State that force is measured in newtons, N, and is a push or pull acting on an object.
- Identify the force perpendicular to the surface when using p = F ÷ A.
- Rearrange p = F ÷ A to F = p × A and substitute pressure in Pa and area in m².
- Calculate weight as a force using W = m × g and use it as F in pressure problems.
- Recognise that changing the contact area changes pressure but not the applied force.
Common mistakes and how to put them right
Treating mass in kilograms as the force in newtons.
Put it right by calculating weight with W = m × g before substituting into p = F ÷ A.
Using a slanted or total surface area instead of the area perpendicular to the force.
Put it right by identifying the contact area normal to the push or pull.
Quoting force in pascals or pressure in newtons.
Put it right by matching each quantity to its unit, N for force and Pa for pressure.
Exam tips
- Underline the force and the area in the question, then check that the force is perpendicular to the area before calculating.
- Show the rearrangement F = p × A explicitly when force is the unknown, and keep units with substituted values.
- If mass is given, calculate weight first using g = 9.8 N/kg and state that this weight is the force acting on the surface.
Try the lesson
The start of the Pressure in a fluid 1: force, F, in newtons, N lesson, made from checked teaching notes for this specification point.
Have a go first
A pressure of 20 Pa acts on an area of 3.0 m². What is the force in newtons?
Listen
Force, F, is a push or pull measured in newtons, N, and in pressure calculations it is the force acting perpendicular to a surface. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s². In p = F ÷ A, F is the total force normal to the area, so a 600 N person standing on both feet exerts 600 N on the floor whatever the area. If the area changes, the force stays the same but the pressure changes. To find force from pressure, rearrange to F = p × A, keeping area in m² so the answer is in N. Weight is a force and is calculated using W = m × g, where g is about 9.8 N/kg on Earth. Always identify which force acts perpendicular to the surface before substituting into the pressure equation. A strong answer earns marks for these points. State that force is measured in newtons, N, and is a push or pull acting on an object. Identify the force perpendicular to the surface when using p = F ÷ A. Rearrange p = F ÷ A to F = p × A and substitute pressure in Pa and area in m². Calculate weight as a force using W = m × g and use it as F in pressure problems. A common slip: Treating mass in kilograms as the force in newtons. Instead, calculating weight with W = m × g before substituting into p = F ÷ A. Exam tip: Underline the force and the area in the question, then check that the force is perpendicular to the area before calculating.
Key words
Say what each word means before you turn the card.
Force
Try answering aloud or in your head before turning the card.
Newton
Try answering aloud or in your head before turning the card.
Weight
Try answering aloud or in your head before turning the card.
See every definition
- Force
- A push or pull acting on an object, measured in newtons, N.
- Newton
- The SI unit of force; one newton gives a 1 kg mass an acceleration of 1 m/s².
- Weight
- A force calculated using W = m × g, where g is about 9.8 N/kg on Earth.
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