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GCSE level physics exam revision notes all about forces Part
7
Pressure and upthrust in fluids: 7.3
Other cases of floating versus sinking
in fluids (gases/liquids), why most solid metal objects float on mercury & how
to control the depth of your submarine!
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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
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INDEX for physics notes: pressure,
forces, weight, upthrust in fluids
7.3
Other cases of floating versus sinking
in fluids (gases or liquids)
Explaining submarine depth control, iron floating on mercury,
why an iron ship floats, why a helium balloon floats in air, various cases of
floating versus sinking in gases and liquid fluids
(Note the
relative density of
water is 1000 kg/m3 or 1g/cm3)
Floating metals!
Iron nails will float on the much more dense
liquid metal mercury.
Group 1 metals: Lithium - density 535 kg/m3
floats on water, caesium - density 1870 kg/m3, sinks in water.
By changing the upthrust submarines can sink
below the surface of water or rise back to the surface.
Sea water can be pumped into tanks to
increase the weight of the submarine to that greater than the weight of the
volume water it displaces i.e. the weight of the submarine is greater than
the upthrust from the sea water.
Far less dense compressed air can be
pumped in to the water tanks, displacing the water, and decreasing the
weight of the submarine to less than the weight of the volume of sea water
displaced by the submarine. The upthrust force is now greater than weight of
the submarine which can rise to the surface.
By controlling the air and water levels
in these buoyancy tanks you can stabilise the submarine at various depths.
You can also get material with a density
greater than the fluid to float by changing its shape e.g.
You can build a ship of steel (density of
~8x water) that floats on water. The shape of the hull, filled with very low
density air, allows the displacement of water equal to the weight of the
ship. If you consider the whole of the volume of the ship, the average
density will be less than that of water. Any individual steel plate of the
ship will of course sink, as will the ship if it is holed and water flows
in!
Why steel
floats!!!
Don't forget materials float or sink in gases
!!!
 Gases are a fluid and any object less dense than the gas will rise up in the
gas due to the upthrust of the more dense fluid.
e.g. a helium filled balloon rises
upwards through the more dense air of the atmospheres - used by
meteorologists for data measurements in the higher atmosphere.
Carbon dioxide is more dense than air and
will initially sink in air - though eventually, the random movement of
particles mixes everything quite fast - which can't happen with the helium
trapped in a rubber balloon.
INDEX for physics notes: pressure,
forces, weight
and upthrust in fluids
Key points
about
pressure in fluids - what floats or sinks in fluids?
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 examples of why objects sink or float in
a fluid, tailored to the major UK GCSE/IGCSE physics exam boards:
WJEC, CCEA, CIE, AQA, Edexcel, and OCR. These notes focus
on real-world examples, physics principles, and exam-relevant content.
Why Do Objects Sink or Float?
- Key Principle:
An object floats if the upthrust (buoyant force) is
equal to or greater than its weight. It sinks if its weight
exceeds the upthrust.
- Upthrust:
The upward force exerted by a fluid on a submerged object.
- Archimedes’ Principle:
Upthrust = weight of fluid displaced by the object.
- Density Rule:
- If object density < fluid
density → floats
- If object density > fluid
density → sinks
Examples of Floating and Sinking
| Example |
Explanation |
| Wood in Water |
Wood is less dense than water → floats |
| Iron Block in Water |
Iron is denser than water → sinks |
| Iceberg in Seawater |
Ice is less dense than seawater →
floats partially submerged |
| Ship |
Displaces water equal to its weight →
floats despite heavy mass |
| Helium Balloon in Air |
Helium is less dense than air → rises
(floats in gas) |
| Hot Air Balloon |
Heated air is less dense → balloon
rises |
| Polystyrene Block |
Very low density → floats easily in
water |
| Submarine |
Adjusts density by compressing air →
can float or sink |
| Hydrometer |
Floats higher in denser liquids → used
to measure fluid density |
| Lead in Mercury |
Lead sinks in water but floats in
denser mercury |
Typical Exam Board Contents
- Covers density,
upthrust, and
floating/sinking comparisons
- Includes Archimedes’ Principle
and particle models
- Emphasises real-world examples
and pressure-depth
relationships
- Focus on
density and pressure
- Includes practical experiments
and fluid displacement
- Encourages understanding of
force balance
- Detailed treatment of buoyancy,
density, and
upthrust
- Requires explanation of
floating/sinking using
force comparisons
- Includes worked examples
and fluid comparisons
- Triple science includes upthrust,
density, and
pressure
- Focus on floating/sinking
in liquids and gases
- Higher tier includes
pressure-depth relationships
- Covers density,
pressure, and buoyancy
- Includes floating/sinking
and fluid displacement
- Emphasises real-world applications
like ships and balloons
- Includes upthrust,
density, and
pressure differences
- Focus on floating/sinking
and Archimedes’ Principle
- Higher tier includes fluid
comparisons and force
balance
Student Tips for Exam Success
- Memorise key formulas:
- Understand Archimedes’ Principle:
Upthrust = weight of fluid displaced
- Use diagrams
to show forces acting on submerged objects
- Compare densities
to predict float/sink outcomes
- Revise gas examples:
balloons, airships, hot air
- Practice calculations
with density and volume
- Use past papers
to identify how floating/sinking is assessed
Keywords, phrases and learning objectives for
upthrust in fluids
Be able to discuss flotation in terms of weight and
upthrust e.g. submarine depth control, why iron ship can float, why helium
balloon floats - explaining cases of floating versus sinking in
fluids like gases liquids.
Explain why an iron nail floats on mercury!
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INDEX for physics notes: pressure
and upthrust in fluids
INDEX of all my physics notes on FORCES
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Revision notes on explaining examples
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for students taking IGCSE/GCSE level physics examinations, summary
revision notes and key points on explaining examples of sinking versus
floating for students taking the AQA
igcse/gcse physics notes on explaining examples of sinking versus floating, Edexcel gcse
physics notes on explaining examples of sinking versus floating, OCR 21st century GCSE
physics notes on explaining examples of sinking versus floating, OCR gateway
GCSE physics notes on explaining examples of sinking versus floating, WJEC gcse physics notes on
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physics, exam revision notes on
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INDEX for physics notes: pressure,
forces, weight
and upthrust in fluids
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