Where can I learn Density and pressure for Cambridge 9702?

This free lesson on MarkScheme covers syllabus point 4.3 (Density and pressure) for Cambridge A-Level Physics. It includes visual revision, key points, and links to real past-paper questions with mark-scheme marking at markscheme.app/courses/9702/4-3-density-and-pressure.

Is Density and pressure (4.3) on 9702 free to study?

Yes. The Physics 9702 course on MarkScheme is 100% free — including premium visual lessons, exam tips, and past-paper marking. No paywall for Density and pressure.

How do I revise Density and pressure for 9702 exams?

Start with the visual steps on this page, note the key exam points, then attempt a 9702 past-paper question on Density and pressure using MarkScheme's /mark tool. Mark strictly against the official scheme and re-read any marks you lost.

MarkScheme 9702

Opening…

4.3Physics25 min readPREMIUM

Density and pressure

Welcome to the world of Density and Pressure! These fundamental concepts help us understand everything from why ships float to how hydraulic brakes work. We'll explore how mass and volume define density, how force and area create pressure, and the mysterious upward push known as upthrust in fluids. This pilot aligns with Senpai Corner AS notes (2025-2027) for topic 4.3.

BY THE END, YOU CAN…

exam checklist — tick these off

1Define and apply density and pressure, including their units and formulas.
2Calculate hydrostatic pressure and explain its dependence on depth.
3Describe upthrust and apply Archimedes' Principle to solve problems.

STUDY MODE

READING

TYPEFACE

Literata — a reading serif made for e-books; set like a textbook, sits well beside maths.

SIZE

AIRY SPACING

Wider letter and line spacing. The one change with solid evidence for readers who lose their place — worth a try whoever you are.

Pick whatever reads fastest for you — it differs from person to person, by up to a third.

EXPLAIN SIMPLER

The same notes in plain English — no jargon. Key points and exam tips stay.

DIAGRAM NOTES

A small sketch beside each block of notes.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Density and pressure

Cambridge 9702 Paper 2 - Density and pressure (4.3). Senpai Corner diagram-backed pilot with premium structure and live visuals.

1
Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
2
The formula for density is $\rho = \frac{m}{V}$ .
3
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
4
Density is an intrinsic property of a material.

What this topic covers

The official Cambridge syllabus points this lesson works through.

4.3.1
Define and use density
4.3.2
Define and use pressure
4.3.3
Derive, from the definitions of pressure and density, the equation for hydrostatic pressure $Δp = ρgΔh$
4.3.4
Use the equation $Δp = ρgΔh$
4.3.5
Understand that the upthrust acting on an object in a fluid is due to a difference in hydrostatic pressure
4.3.6
Calculate the upthrust acting on an object in a fluid using the equation $F = ρgV$ (Archimedes' principle)

Explore the concept

Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.

Figure PhET simulation1 / 4

Loading simulation…

Fetching from PhET…

Density

Compare block mass in the same volume — denser materials feel heavier.

If the embed does not load, use Open on PhET.

Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)

Step-synced diagram — highlights what to look for in the simulation above.

Density ρ = m/V

Density ρ = m/V — mass per unit volume (kg m⁻³).

20 more simulations for this topic — run them in the Simulations section below

Simulations

Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.

20 simulations · 5 to start with

Start herein this order — each one shows a different piece of the topic

3JCN PhysicsStart here · 19702 4.3
Fluid Pressure
Move a probe to different depths; read pressure and confirm p = rho g h
Why this one: Move the probe down: pressure rises in a straight line with depth at rate ρg, so p = ρgh.
Try this
1. Move the probe to a shallow depth and read the pressure.
2. Double the depth and read again.
3. Check each reading against p = ρgh.
Look for Pressure increases linearly with depth at rate ρg.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
oPhysicsStart here · 29702 4.3 · IB A.2
Fluid Density U-Tube Lab
U-tube with two fluids; use column heights to find an unknown density (explore and lab modes)
Why this one: Two liquids in a U-tube balance where ρ₁h₁ = ρ₂h₂; use the column heights to find the unknown density.
Try this
1. Add the two fluids and read both column heights.
2. Compute the unknown density from the heights.
3. Switch to lab mode and check.
Look for Pressures balance at the interface, so ρ₁h₁ = ρ₂h₂.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
PhETStart here · 39702 4.3
Buoyancy
Weigh blocks in air and in fluid; read the buoyant force and the fluid displaced.
Why this one: Weigh a block in air then in fluid; the lost weight equals the weight of fluid it displaces.
Try this
1. Submerge a 2 kg block fully — compare the scale reading with its weight in air.
2. Change the fluid density (oil → water → honey) — how does the upthrust change?
3. Read the displaced volume and check F = ρ_fluid V g.
Look for Upthrust equals the weight of fluid displaced — the pressure difference top to bottom does the lifting.
Open on PhET
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
3JCN PhysicsStart here · 49702 4.3
Archimedes Prin.
Submerge objects of different densities; read displaced volume and upthrust
Why this one: Upthrust depends on displaced volume and fluid density, not on what the object is made of.
Try this
1. Submerge an object and read the displaced volume and upthrust.
2. Submerge a denser object of the same volume and compare the upthrust.
3. Pick an object less dense than the fluid and watch it float.
Look for Upthrust equals the weight of fluid displaced, independent of the object's own density.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
SimuPhysicsStart here · 59702 4.3
Hydrostatic Paradox
Differently shaped vessels filled to the same height stand side by side; compare the gauge readings at their bases
Why this one: Odd-shaped vessels filled to one height read the same base pressure: only depth matters.
Try this
1. Fill all vessels to the same height and read the gauges.
2. Change the height and read them again.
Look for Pressure at the base depends on the liquid height, not on the shape or volume of the vessel.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)

More simulations15 more on this topic — core ones first

PhETCoreJava · best on a laptop9702 4.3 · IB A.2
Fluid Pressure and Flow
Measure pressure at depth in fluids, then explore flow through pipes and a water tower.
Try this
1. Lower the gauge into the tank — pressure rises steadily with depth.
2. Switch the fluid density — the same depth reads a different pressure.
3. On the Flow screen, narrow the pipe — the fluid speeds up.
Look for Δp = ρgΔh; in a pipe, a narrower section carries faster flow.
Open on PhET
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
oPhysicsCore9702 4.3 · IB A.2
Buoyancy
Adjust fluid density, object density and viscosity; run to see sinking, floating and the buoyant force
Try this
1. Set the object denser than the fluid and run.
2. Make it less dense and run again.
3. Raise the viscosity.
Look for The buoyant force equals the weight of fluid displaced, so the object floats when it is less dense than the fluid.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
oPhysicsCore9702 4.3 · IB A.2
Density Lab Using Buoyancy
Lab: weigh an object in air and submerged in a known fluid, compute its density and check
Try this
1. Weigh the object in air.
2. Submerge it and weigh again.
3. Compute the density from the two readings and check.
Look for The loss of weight equals the weight of fluid displaced, giving the object’s volume and hence its density.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
The Physics ClassroomCore9702 4.3 · IB A.2
Buoyancy Sim
Place an object in a fluid and see the forces at play; set its density to decide whether it floats or sinks
Try this
1. Place a low-density object and watch it float.
2. Increase its density until it sinks.
3. Compare the buoyant force with the weight.
Look for An object floats when the buoyant force equals its weight.
Open on The Physics Classroom
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
SimuPhysicsCore9702 4.3
Liquid Displacement in U Tube
Choose the densities of two liquids that do not mix and see the U-tube column heights adjust until the pressures balance
Try this
1. Set the two densities equal and read the heights.
2. Make one liquid denser and compare the heights.
Look for The denser liquid stands lower, so that density times height matches on both sides.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
SimuPhysicsCore9702 4.3
Pressure vs Height in Non-Uniform Containers
Probe the pressure at any point inside oddly shaped containers and plot it against depth
Try this
1. Probe at several depths and plot the pressure.
2. Probe at the same depth where the container widens.
3. Compare the graph across containers.
Look for Pressure rises in a straight line with depth regardless of the container's width.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)

Key formulas

Tap any symbol to reveal exactly what it means and its units.

KEY RELATION

$\rho = \frac{m}{V}$

Tap a symbol — great for exam definitions

KEY RELATION

$P = \frac{F}{A}$

Tap a symbol — great for exam definitions

KEY RELATION

$\Delta p = \rho g \Delta h$

Tap a symbol — great for exam definitions

KEY RELATION

$F_B = \rho_{fluid} g V_{displaced}$

Tap a symbol — great for exam definitions

Full topic notes

Formal explanation with the rigour you need for the exam.

Understanding Density: How Packed is it?

Density is a measure of how much 'stuff' (mass) is squeezed into a certain amount of space (volume). Imagine two boxes of the same size; one filled with feathers and the other with rocks. The box of rocks has a much higher density because there's more mass in the same volume. To measure the density of a regular solid, you can measure its dimensions to calculate volume and use a balance for its mass. For an irregular solid, you can find its volume by submerging it in a measuring cylinder with water and observing the volume of water displaced.

$\rho = \frac{m}{V}$

Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
The formula for density is $\rho = \frac{m}{V}$ .
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
Density is an intrinsic property of a material.

Not clear?

Pressure: Force Distributed

Pressure describes how a force is spread out over an area. If you push on a wall with your flat hand, the force is distributed over a large area, resulting in low pressure. If you push with just your fingertip, the same force concentrated on a tiny area creates much higher pressure. It's important to note that while force is a vector, pressure is a scalar quantity; it has magnitude but no direction. At any point within a fluid, pressure is exerted equally in all directions.

$P = \frac{F}{A}$

Pressure ( $P$ ) is the perpendicular force ( $F$ ) exerted per unit area ( $A$ ).
The formula for pressure is $P = \frac{F}{A}$ .
The SI unit for pressure is the Pascal (Pa), which is equivalent to 1 N m^{-2}.
Pressure is a scalar quantity, meaning it has magnitude but no direction.

Not clear?

Pressure in Fluids: Diving Deeper

When you dive into a swimming pool, you feel the pressure increase as you go deeper. This is because the fluid above you is exerting a weight. The deeper you go, the more fluid is above you, and thus, the greater the pressure. This hydrostatic pressure can be derived by considering a vertical column of fluid of height $\Delta h$ and cross-sectional area $A$ . The volume of this column is $V = A \Delta h$ , and its mass is $m = \rho V = \rho A \Delta h$ . The weight of the column is $W = mg = \rho A \Delta h g$ . The pressure exerted by this weight is $p = \frac{W}{A} = \frac{\rho A \Delta h g}{A} = \rho g \Delta h$ . The total pressure at a depth is this gauge pressure plus the atmospheric pressure at the surface.

$\Delta p = \rho g \Delta h$

Fluid pressure increases linearly with depth.
The formula for the change in pressure (gauge pressure) is $\Delta p = \rho g \Delta h$ .
Total (absolute) pressure at a depth is the sum of gauge pressure and atmospheric pressure: $p_{total} = p_{atm} + \rho g h$ .
At any given depth, pressure in a static fluid acts equally in all directions.

Not clear?

Upthrust and Archimedes' Principle: The Floating Secret

Have you ever tried to push a ball underwater? It feels like something is pushing it back up - that's upthrust! This buoyant force arises because the pressure at the bottom of a submerged object is greater than at its top, creating an overall upward push. According to Archimedes' principle, this upthrust is equal to the weight of the fluid the object displaces. An object's fate in a fluid is determined by the balance between its weight and the upthrust. If the object's density is less than the fluid's density, it will float. If its density is greater, it will sink. If the densities are equal, it will remain suspended at any depth it's placed.

$F_B = \rho_{fluid} g V_{displaced}$

Upthrust, or buoyant force ( $F_B$ ), is the upward force exerted by a fluid on a submerged or floating object.
It is caused by the pressure difference between the bottom and top surfaces of the object.
Archimedes' Principle: Upthrust is equal to the weight of the fluid displaced by the object.
An object floats if its average density is less than the fluid's density. It sinks if its average density is greater.

Not clear?

Worked examples

See the formulas applied — reveal one step at a time, like the exam.

reveal slowly — mark each step

EXAMPLE 1

A submarine is at a depth of 250 m in seawater. The density of seawater is 1030 kg m^{-3}. The submarine has a circular viewing window with a radius of 0.20 m. Calculate (a) the gauge pressure on the outside of the window, and (b) the total force exerted on the window by the water. (Use $g = 9.81 \text{ m s}^{-2}$ ). Give your answers to two significant figures.

1
(a) Calculate the gauge pressure:

EXAMPLE 2

A block of wood with dimensions 0.10 m x 0.10 m x 0.10 m has a mass of 0.80 kg. It is fully submerged in water (density 1000 kg m^{-3}). Calculate: (a) the density of the wood, (b) the upthrust acting on the wood, and (c) the net force acting on the wood (take $g = 9.81 \text{m s}^{-2}$ ).

1
Calculate the volume of the wood:

How it all connects

The big idea sits in the middle — tap a linked idea to explore the link.

MAIN IDEADensity and pressure

Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.

Glossary

Key terms for this topic — skim now; the Check step will test them.

total pressure: The total pressure (or absolute pressure) is the sum of the fluid's hydrostatic pressure (gauge pressure) and the atmospheric pressure above it: $p_{total} = p_{atm} + \rho g h$ .

10Apply

Quick check

Write your answer first, then compare it with the model one — the gap is what you would have lost.

0 of 6 answered

11Understand

Teach it back

If you can explain it simply, you own it — gaps here are marks you’d lose.

Teach it back

Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.

Your explanation — no notes

12Remember

Revision flashcards

Guess first, then flip — retrieval beats re-reading.

Your guess — write it before you look

1 / 10 · ← → space

Key takeaways

Review these before you close the topic — retrieval beats re-reading.

close the tab only after these stick

Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
The formula for density is $\rho = \frac{m}{V}$ .
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
Density is an intrinsic property of a material.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Question 1 of 29702/23 · Q1(d)(i)

PhysicsMay/June 2024

(d) (i)

The density of the liquid is 920 kg m⁻³. Show that the upthrust acting on the sphere is 1.0 N.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/23/M/J/24

Question 2 of 29702/23 · Q1(d)(ii)

PhysicsMay/June 2024

(d) (ii)

Calculate the mass of the sphere.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/23/M/J/24

Extra simulations & links

PhET, GeoGebra and other curated tools — open in a new tab.

Frequently asked

Why does pressure increase with depth in a fluid?

What is the key difference between density and pressure?

How does Archimedes' Principle explain why objects float or sink?

Checkpoint

One marked question is worth ten re-reads — close the loop before you move on.

Checkpoint

Reading it isn’t knowing it — prove it.

Before you move on: do 9702/23 · Q1(d)(i) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

Attempt & get marked →Takes about a minute · you’ll land back on this lesson

← PREVIOUS4.2 Equilibrium of Forces NEXT →5.1 Energy conservation

Discuss Density and pressure

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Revise Density and pressure for Cambridge 9702 (4.3)

Looking for free Physics 9702 notes on Density and pressure? This A-Level lesson covers official syllabus point 4.3 on Paper 1: Multiple Choice / Paper 2: AS Level Structured Questions. Use the visual steps, key terms, and quick checks below — then practise a real 9702 past-paper question with mark-scheme marking. Built for students searching "Density and pressure 9702 revision", "9702 4.3 notes", or a free alternative to paid note sites.

Full Physics 9702 course Mark a past paper →9702 subject hub

4.3Physics25 min readPREMIUM

Density and pressure

BY THE END, YOU CAN…

exam checklist — tick these off

1Define and apply density and pressure, including their units and formulas.
2Calculate hydrostatic pressure and explain its dependence on depth.
3Describe upthrust and apply Archimedes' Principle to solve problems.

STUDY MODE

READING

TYPEFACE

Literata — a reading serif made for e-books; set like a textbook, sits well beside maths.

SIZE

AIRY SPACING

Wider letter and line spacing. The one change with solid evidence for readers who lose their place — worth a try whoever you are.

Pick whatever reads fastest for you — it differs from person to person, by up to a third.

EXPLAIN SIMPLER

The same notes in plain English — no jargon. Key points and exam tips stay.

DIAGRAM NOTES

A small sketch beside each block of notes.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Density and pressure

Cambridge 9702 Paper 2 - Density and pressure (4.3). Senpai Corner diagram-backed pilot with premium structure and live visuals.

1
Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
2
The formula for density is $\rho = \frac{m}{V}$ .
3
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
4
Density is an intrinsic property of a material.

What this topic covers

The official Cambridge syllabus points this lesson works through.

4.3.1
Define and use density
4.3.2
Define and use pressure
4.3.3
Derive, from the definitions of pressure and density, the equation for hydrostatic pressure $Δp = ρgΔh$
4.3.4
Use the equation $Δp = ρgΔh$
4.3.5
Understand that the upthrust acting on an object in a fluid is due to a difference in hydrostatic pressure
4.3.6
Calculate the upthrust acting on an object in a fluid using the equation $F = ρgV$ (Archimedes' principle)

Explore the concept

Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.

Figure PhET simulation1 / 4

Loading simulation…

Fetching from PhET…

Density

Compare block mass in the same volume — denser materials feel heavier.

If the embed does not load, use Open on PhET.

Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)

Step-synced diagram — highlights what to look for in the simulation above.

Density ρ = m/V

Density ρ = m/V — mass per unit volume (kg m⁻³).

20 more simulations for this topic — run them in the Simulations section below

Simulations

Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.

20 simulations · 5 to start with

Start herein this order — each one shows a different piece of the topic

3JCN PhysicsStart here · 19702 4.3
Fluid Pressure
Move a probe to different depths; read pressure and confirm p = rho g h
Why this one: Move the probe down: pressure rises in a straight line with depth at rate ρg, so p = ρgh.
Try this
1. Move the probe to a shallow depth and read the pressure.
2. Double the depth and read again.
3. Check each reading against p = ρgh.
Look for Pressure increases linearly with depth at rate ρg.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
oPhysicsStart here · 29702 4.3 · IB A.2
Fluid Density U-Tube Lab
U-tube with two fluids; use column heights to find an unknown density (explore and lab modes)
Why this one: Two liquids in a U-tube balance where ρ₁h₁ = ρ₂h₂; use the column heights to find the unknown density.
Try this
1. Add the two fluids and read both column heights.
2. Compute the unknown density from the heights.
3. Switch to lab mode and check.
Look for Pressures balance at the interface, so ρ₁h₁ = ρ₂h₂.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
PhETStart here · 39702 4.3
Buoyancy
Weigh blocks in air and in fluid; read the buoyant force and the fluid displaced.
Why this one: Weigh a block in air then in fluid; the lost weight equals the weight of fluid it displaces.
Try this
1. Submerge a 2 kg block fully — compare the scale reading with its weight in air.
2. Change the fluid density (oil → water → honey) — how does the upthrust change?
3. Read the displaced volume and check F = ρ_fluid V g.
Look for Upthrust equals the weight of fluid displaced — the pressure difference top to bottom does the lifting.
Open on PhET
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
3JCN PhysicsStart here · 49702 4.3
Archimedes Prin.
Submerge objects of different densities; read displaced volume and upthrust
Why this one: Upthrust depends on displaced volume and fluid density, not on what the object is made of.
Try this
1. Submerge an object and read the displaced volume and upthrust.
2. Submerge a denser object of the same volume and compare the upthrust.
3. Pick an object less dense than the fluid and watch it float.
Look for Upthrust equals the weight of fluid displaced, independent of the object's own density.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
SimuPhysicsStart here · 59702 4.3
Hydrostatic Paradox
Differently shaped vessels filled to the same height stand side by side; compare the gauge readings at their bases
Why this one: Odd-shaped vessels filled to one height read the same base pressure: only depth matters.
Try this
1. Fill all vessels to the same height and read the gauges.
2. Change the height and read them again.
Look for Pressure at the base depends on the liquid height, not on the shape or volume of the vessel.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)

More simulations15 more on this topic — core ones first

PhETCoreJava · best on a laptop9702 4.3 · IB A.2
Fluid Pressure and Flow
Measure pressure at depth in fluids, then explore flow through pipes and a water tower.
Try this
1. Lower the gauge into the tank — pressure rises steadily with depth.
2. Switch the fluid density — the same depth reads a different pressure.
3. On the Flow screen, narrow the pipe — the fluid speeds up.
Look for Δp = ρgΔh; in a pipe, a narrower section carries faster flow.
Open on PhET
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
oPhysicsCore9702 4.3 · IB A.2
Buoyancy
Adjust fluid density, object density and viscosity; run to see sinking, floating and the buoyant force
Try this
1. Set the object denser than the fluid and run.
2. Make it less dense and run again.
3. Raise the viscosity.
Look for The buoyant force equals the weight of fluid displaced, so the object floats when it is less dense than the fluid.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
oPhysicsCore9702 4.3 · IB A.2
Density Lab Using Buoyancy
Lab: weigh an object in air and submerged in a known fluid, compute its density and check
Try this
1. Weigh the object in air.
2. Submerge it and weigh again.
3. Compute the density from the two readings and check.
Look for The loss of weight equals the weight of fluid displaced, giving the object’s volume and hence its density.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
The Physics ClassroomCore9702 4.3 · IB A.2
Buoyancy Sim
Place an object in a fluid and see the forces at play; set its density to decide whether it floats or sinks
Try this
1. Place a low-density object and watch it float.
2. Increase its density until it sinks.
3. Compare the buoyant force with the weight.
Look for An object floats when the buoyant force equals its weight.
Open on The Physics Classroom
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
SimuPhysicsCore9702 4.3
Liquid Displacement in U Tube
Choose the densities of two liquids that do not mix and see the U-tube column heights adjust until the pressures balance
Try this
1. Set the two densities equal and read the heights.
2. Make one liquid denser and compare the heights.
Look for The denser liquid stands lower, so that density times height matches on both sides.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
SimuPhysicsCore9702 4.3
Pressure vs Height in Non-Uniform Containers
Probe the pressure at any point inside oddly shaped containers and plot it against depth
Try this
1. Probe at several depths and plot the pressure.
2. Probe at the same depth where the container widens.
3. Compare the graph across containers.
Look for Pressure rises in a straight line with depth regardless of the container's width.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)

Key formulas

Tap any symbol to reveal exactly what it means and its units.

KEY RELATION

$\rho = \frac{m}{V}$

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KEY RELATION

$P = \frac{F}{A}$

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KEY RELATION

$\Delta p = \rho g \Delta h$

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KEY RELATION

$F_B = \rho_{fluid} g V_{displaced}$

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Full topic notes

Formal explanation with the rigour you need for the exam.

Understanding Density: How Packed is it?

$\rho = \frac{m}{V}$

Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
The formula for density is $\rho = \frac{m}{V}$ .
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
Density is an intrinsic property of a material.

Not clear?

Pressure: Force Distributed

$P = \frac{F}{A}$

Pressure ( $P$ ) is the perpendicular force ( $F$ ) exerted per unit area ( $A$ ).
The formula for pressure is $P = \frac{F}{A}$ .
The SI unit for pressure is the Pascal (Pa), which is equivalent to 1 N m^{-2}.
Pressure is a scalar quantity, meaning it has magnitude but no direction.

Not clear?

Pressure in Fluids: Diving Deeper

$\Delta p = \rho g \Delta h$

Fluid pressure increases linearly with depth.
The formula for the change in pressure (gauge pressure) is $\Delta p = \rho g \Delta h$ .
Total (absolute) pressure at a depth is the sum of gauge pressure and atmospheric pressure: $p_{total} = p_{atm} + \rho g h$ .
At any given depth, pressure in a static fluid acts equally in all directions.

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Upthrust and Archimedes' Principle: The Floating Secret

$F_B = \rho_{fluid} g V_{displaced}$

Upthrust, or buoyant force ( $F_B$ ), is the upward force exerted by a fluid on a submerged or floating object.
It is caused by the pressure difference between the bottom and top surfaces of the object.
Archimedes' Principle: Upthrust is equal to the weight of the fluid displaced by the object.
An object floats if its average density is less than the fluid's density. It sinks if its average density is greater.

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Worked examples

See the formulas applied — reveal one step at a time, like the exam.

reveal slowly — mark each step

EXAMPLE 1

1
(a) Calculate the gauge pressure:

EXAMPLE 2

1
Calculate the volume of the wood:

How it all connects

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MAIN IDEADensity and pressure

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Glossary

Key terms for this topic — skim now; the Check step will test them.

total pressure: The total pressure (or absolute pressure) is the sum of the fluid's hydrostatic pressure (gauge pressure) and the atmospheric pressure above it: $p_{total} = p_{atm} + \rho g h$ .

10Apply

Quick check

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0 of 6 answered

11Understand

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12Remember

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Your guess — write it before you look

1 / 10 · ← → space

Key takeaways

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close the tab only after these stick

Density ( $\rho$ ) is the mass ( $m$ ) per unit volume ( $V$ ) of a substance.
The formula for density is $\rho = \frac{m}{V}$ .
The standard SI unit for density is kilograms per cubic meter (kg m^{-3}).
Density is an intrinsic property of a material.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Question 1 of 29702/23 · Q1(d)(i)

PhysicsMay/June 2024

(d) (i)

The density of the liquid is 920 kg m⁻³. Show that the upthrust acting on the sphere is 1.0 N.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/23/M/J/24

Question 2 of 29702/23 · Q1(d)(ii)

PhysicsMay/June 2024

(d) (ii)

Calculate the mass of the sphere.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/23/M/J/24

Extra simulations & links

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Frequently asked

Why does pressure increase with depth in a fluid?

What is the key difference between density and pressure?

How does Archimedes' Principle explain why objects float or sink?

Checkpoint

One marked question is worth ten re-reads — close the loop before you move on.

Checkpoint

Reading it isn’t knowing it — prove it.

Before you move on: do 9702/23 · Q1(d)(i) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

Attempt & get marked →Takes about a minute · you’ll land back on this lesson

← PREVIOUS4.2 Equilibrium of Forces NEXT →5.1 Energy conservation

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Revise Density and pressure for Cambridge 9702 (4.3)

In simple terms

What this topic covers

Explore the concept

Simulations

Fluid Pressure

Fluid Density U-Tube Lab

Buoyancy

Archimedes Prin.

Hydrostatic Paradox

Fluid Pressure and Flow

Buoyancy

Density Lab Using Buoyancy

Buoyancy Sim

Liquid Displacement in U Tube

Pressure vs Height in Non-Uniform Containers

Key formulas

Full topic notes

Understanding Density: How Packed is it?

Pressure: Force Distributed

Pressure in Fluids: Diving Deeper

Upthrust and Archimedes' Principle: The Floating Secret

Worked examples

How it all connects

Glossary

Quick check

Teach it back

Revision flashcards

Key takeaways

Practice — then mark it

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Density and pressure

Revise Density and pressure for Cambridge 9702 (4.3)

In simple terms

What this topic covers

Explore the concept

Simulations

Fluid Pressure

Fluid Density U-Tube Lab

Buoyancy

Archimedes Prin.

Hydrostatic Paradox

Fluid Pressure and Flow

Buoyancy

Density Lab Using Buoyancy

Buoyancy Sim

Liquid Displacement in U Tube

Pressure vs Height in Non-Uniform Containers

Key formulas

Full topic notes

Understanding Density: How Packed is it?

Pressure: Force Distributed

Pressure in Fluids: Diving Deeper

Upthrust and Archimedes' Principle: The Floating Secret

Worked examples

How it all connects

Glossary

Quick check

Teach it back

Revision flashcards

Key takeaways

Practice — then mark it

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Density and pressure