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School-college Physics Notes: Forces 6.1 Particle theory and properties of fluids

GCSE level physics exam revision notes all about forces Part 6

Forces and pressure: 6.1

Particle theory of states of matter with respect to density and pressure in solids and fluids due to their weight and depth

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics[forces-6- page updated Mar 16th 2026 *]

[KEY POINTS and learning objectives for this page, after initial notes]

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Sub-index physics notes on 6. Forces and Pressure in fluids & calculations

6.1 Particle theory - revision of states of matter and density

Gas and liquid fluid particles are free to move around from place to place, but not in solids!

What is a fluid?

Fluids are materials that flow, usually gases or liquids (a very fine powder can flow like a fluid and a liquid can flow with fine solid particles in it e.g. a mud slide).

Therefore fluids have virtually no tensile strength, a very viscous fluid ('very sticky') has a very small tensile strength.

In solids, the particles can just vibrate from a fixed position and cannot move to another position in the lattice of atoms, but the bonding is strong, so solids exhibit a wide range of tensile strength and are difficult to either compress, stretch or bend.

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very low density FLUID high density FLUID highest density, but NOT a FLUID

Solids have the highest density, the particles are the closest together.

In fluids the inter-particle attractive forces are sufficiently weak to prevent a solid forming, allowing free random movement of the molecules of a liquid or gas.

Because of the weaker inter-particle force, the particles of gas will spread out to fill any space available giving gases by far the lowest density of the three states of matter.

When a solid is

The particles in a liquid are held much closer together because of greater inter-particle forces giving liquids a much greater density than gases.

Apart from water, liquid densities of materials are usually a few % less than that of their solid form.

The density of liquids is so high, with so little space between the particles, they are almost impossible to compress to a smaller volume.

Gases have so much space between the particles that they are readily compressed to a smaller volume.

The closer together the particles are, the more compact the substance is, the greater density of it.

See also Particle theory models of gases, liquids and solids and the particle diagram above.

 

Liquids have a uniform density (same throughout its bulk) which only increases very slightly under extremely high pressures because there is so little free space to squeeze the molecules into.

However, there is considerable space between gaseous particles and its is relatively easy to compress the particles closer together e.g a bicycle pump filled with air or water.

The air is easily compressed and the water isn't. The water can ruin the pump but you can't compress the water in it. High pressure water pistols rely on compressed air NOT compressed water.

 

Liquid fluids have a much greater density than gaseous fluids

So, for similar depths (or heights) of gas and liquid, liquids will create a much greater pressure because of the greater weight (due to gravity) of substance acting on the same surface area.

The greater the density of a material, the greater the number of collisions can take place, creating a greater pressure.

This concept is most applicable to gases, which are so easily compressed under pressure, so considerably increasing their density - this happens with the Earth's atmosphere.

Therefore do NOT assume gases cannot create a pressure due to their weight.

Also, the greater the column of fluid e.g. water, the greater the pressure created - the greater weight acting on a given area.

See later section on pressure in liquids.

 

See also

Particle models of gases–liquids–solids, explaining properties, state changes (GCSE chemistry)

The density of materials and the particle model of matter (GCSE physics notes)

Gas law calculations - P, V and T relationships (GCSE chemistry notes)

Index physics Forces notes 6. Forces & pressure in fluids, calculations

Key points about forces in fluids - gases and liquids

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured set of summary revision notes on fluids tailored to the major UK GCSE/IGCSE physics exam boards: WJEC, CCEA, CIE, AQA, Edexcel, and OCR. I’ve also included exam tips to help you tackle fluid-related questions with confidence.

What Is a Fluid?

Definition:

A fluid is any substance that can flow.

This includes liquids and gases, as both have the ability to change shape and move under the influence of forces.

Key Properties:

  • No fixed shape – takes the shape of its container
  • Can be compressed (gases more than liquids)
  • Exerts pressure in all directions
  • Can flow from high to low pressure areas or just flow under a gravitational field.

Typical Core Concepts Across Exam Boards

Concept Description
Density Density = mass / volume (ρ = m/V)
Pressure in Fluids Pressure = Force / Area (P = F/A)
Pressure in Liquids P = hρg – increases with depth
Atmospheric Pressure Caused by air molecules colliding with surfaces
Upthrust (Buoyancy) Upward force due to displaced fluid; linked to Archimedes’ Principle
Hydraulic Systems Use incompressible liquids to transmit force via pressure
Viscosity Resistance to flow – thicker fluids have higher viscosity
Streamlined Flow versus Turbulence Smooth versus chaotic flow patterns

Typical Exam Board-Specific Contents

  • Focus on pressure in liquids and gases
  • Applications: hydraulics, atmospheric pressure, diving and altitude
  • Includes density, pressure, and hydraulic systems
  • Often links to real-world applications like braking systems
  • Strong emphasis on fluid pressure, upthrust, and density
  • May include experimental methods for measuring density
  • Covers fluid pressure, density, and hydraulics
  • Includes practical investigations and data interpretation
  • Focus on pressure in liquids, density, and buoyancy
  • Often includes numerical calculations and unit conversions
  • Covers fluid properties, pressure, density, and upthrust
  • Includes Archimedes’ Principle and hydraulic systems

Student Tips for Exam Success

  • Memorise key formulas: especially for pressure and density
  • Practice unit conversions: e.g. cm³ to m³, g to kg
  • Use diagrams: to show pressure changes with depth or hydraulic systems
  • Understand real-world applications: like car brakes, submarines, and barometers
  • Show working clearly: especially in multi-step calculations

Keywords, phrases and learning objectives for forces involving pressure situations

Be able to explain using particle model theory, the relative density and pressure in gases, liquids and solids due to their weight.

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INDEX for physics notes on 6. Forces and Pressure

INDEX of all my physics notes on FORCES

INDEX of all my physics notes on FORCES and MOTION

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