Nine Tenths of Every Iceberg Is Hiding Under the Water
Learn to define density and relative density and convert between units, find relative density of a solid and a liquid by Archimedes' principle, predict whether a body floats or sinks, and calculate the fraction submerged.
Why is only a tenth of an iceberg visible above the water?
An iceberg floats with almost all of itself hidden. The part you can see is roughly a tenth of the whole, and the reason is a single ratio.
Ice has a density of about and water . A floating body sinks until the water it has pushed aside weighs exactly as much as the body does — so the submerged fraction is
Ninety per cent under, ten per cent showing. And nothing about the iceberg's size entered that calculation — a small floating chunk and a mountain of ice both show the same tenth.
The ratio has a name: the relative density of ice. It is the single number that decides whether a body floats, how much of it sinks, and in which liquids it will do either.
So the whole of this page reduces to comparing two densities. It covers the second part of the ICSE Class 9 Physics chapter on upthrust and floatation — density and relative density, measuring them by Archimedes' principle, the principle of floatation, and its applications.
Ice has a density of about and water . A floating body sinks until the water it has pushed aside weighs exactly as much as the body does — so the submerged fraction is
Ninety per cent under, ten per cent showing. And nothing about the iceberg's size entered that calculation — a small floating chunk and a mountain of ice both show the same tenth.
The ratio has a name: the relative density of ice. It is the single number that decides whether a body floats, how much of it sinks, and in which liquids it will do either.
So the whole of this page reduces to comparing two densities. It covers the second part of the ICSE Class 9 Physics chapter on upthrust and floatation — density and relative density, measuring them by Archimedes' principle, the principle of floatation, and its applications.
Formula
What is relative density, and how do you convert the units?
Density is mass per unit volume; relative density is how many times denser a substance is than water.
- Density has the SI unit , and the CGS unit
- Relative density has NO unit — it is a ratio of two densities, so the units cancel
The conversion. Since g kg and ,
**So multiply by to go from to **, and divide by to come back.
Worked conversions.
- Water:
- Mercury:
- Ice:
- Iron:
- Cork:
Worked example 1 — density from mass and volume. A block of mass g and volume :
Worked example 2 — mass from density. A brass ornament of volume made of a metal of density :
Worked example 3 — relative density. Iron has , so
Notice the pattern. The relative density is numerically the same as the density expressed in — because water's density is exactly in those units. **So RD and carry the same number and mean different things, and one of them must not be given a unit.
Worked example 4 — density from RD.** A liquid has RD , so
Writing a unit after a relative density is a marked error. RD is a pure number, so *the relative density is * is wrong even though the digits are right. The absence of the unit is the whole point of the quantity — it means the same thing to anyone, in any unit system, which is why it is the convenient number to quote and to compare.
- Density has the SI unit , and the CGS unit
- Relative density has NO unit — it is a ratio of two densities, so the units cancel
The conversion. Since g kg and ,
**So multiply by to go from to **, and divide by to come back.
Worked conversions.
- Water:
- Mercury:
- Ice:
- Iron:
- Cork:
Worked example 1 — density from mass and volume. A block of mass g and volume :
Worked example 2 — mass from density. A brass ornament of volume made of a metal of density :
Worked example 3 — relative density. Iron has , so
Notice the pattern. The relative density is numerically the same as the density expressed in — because water's density is exactly in those units. **So RD and carry the same number and mean different things, and one of them must not be given a unit.
Worked example 4 — density from RD.** A liquid has RD , so
Writing a unit after a relative density is a marked error. RD is a pure number, so *the relative density is * is wrong even though the digits are right. The absence of the unit is the whole point of the quantity — it means the same thing to anyone, in any unit system, which is why it is the convenient number to quote and to compare.
How do you find relative density using Archimedes' principle?
Weigh the body in air and in water, and divide the air weight by the loss. The upthrust does the measuring for you.
For a solid denser than water. Since is the weight of water displaced, and the body has the same volume,
Worked example 1. A solid weighs gf in air and gf in water:
so its density is , or .
Worked example 2 — in newtons. A solid weighs N in air and N in water:
The unit of weight does not matter, since it cancels in the ratio — gram-force, newton or kilogram-force all give the same RD. That cancellation is why the method is quoted as a ratio of weights rather than as a density calculation needing a volume.
Worked example 3 — a heavier solid. A metal weighs gf in air and gf in water:
which identifies it as iron.
For a liquid. Use the same solid in both the liquid and in water, so its volume cancels:
Worked example 4. A solid weighs gf in air, gf in water and gf in a liquid:
so the liquid's density is — lighter than water, which is why the loss was smaller.
Worked example 5 — a denser liquid. The same solid weighs gf in a third liquid:
Worked example 6 — the full chain. For the solid of worked example 4, find its own RD too:
So one set of three weighings gives the relative density of both the solid and the liquid. That efficiency is the reason the method is used, and the three readings — in air, in water, in the liquid — are what an examination question supplies.
The solid must sink in both liquids for this to work. If the solid floats in the liquid it does not displace its own volume, the volume no longer cancels, and the formula fails. So a dense sinker is chosen deliberately, and a question offering a cork is testing whether you noticed.
For a solid denser than water. Since is the weight of water displaced, and the body has the same volume,
Worked example 1. A solid weighs gf in air and gf in water:
so its density is , or .
Worked example 2 — in newtons. A solid weighs N in air and N in water:
The unit of weight does not matter, since it cancels in the ratio — gram-force, newton or kilogram-force all give the same RD. That cancellation is why the method is quoted as a ratio of weights rather than as a density calculation needing a volume.
Worked example 3 — a heavier solid. A metal weighs gf in air and gf in water:
which identifies it as iron.
For a liquid. Use the same solid in both the liquid and in water, so its volume cancels:
Worked example 4. A solid weighs gf in air, gf in water and gf in a liquid:
so the liquid's density is — lighter than water, which is why the loss was smaller.
Worked example 5 — a denser liquid. The same solid weighs gf in a third liquid:
Worked example 6 — the full chain. For the solid of worked example 4, find its own RD too:
So one set of three weighings gives the relative density of both the solid and the liquid. That efficiency is the reason the method is used, and the three readings — in air, in water, in the liquid — are what an examination question supplies.
The solid must sink in both liquids for this to work. If the solid floats in the liquid it does not displace its own volume, the volume no longer cancels, and the formula fails. So a dense sinker is chosen deliberately, and a question offering a cork is testing whether you noticed.
How do you tell whether a body will float, sink or stay suspended?
Compare the body's density with the liquid's. Nothing else is needed.
- — the body sinks
- — the body stays suspended, fully submerged but neither rising nor falling
- — the body floats, with part of it above the surface
Why the comparison decides it. Fully immerse a body of volume . Then
and the and are common to both, so whichever density is larger gives the larger force. **If the body is pushed up and rises; if it sinks; if they are equal it stays put.
The principle of floatation: a floating body displaces a weight of liquid exactly equal to its own weight. It sinks only far enough to find that much upthrust and no further.
Worked example 1 — classifying.** In water ():
- iron, — sinks
- ice, — floats
- cork, — floats
- a body of exactly — suspended
- mercury, — sinks
Worked example 2 — the same body in two liquids. A body of density :
- in water (): denser, so it sinks
- in a salt solution of : less dense, so it floats
One body, two outcomes — which is why does it float is an incomplete question until the liquid is named.
The fraction submerged. For a floating body, weight equals upthrust:
Worked example 3 — ice in water. , so ** per cent submerged** and per cent showing.
Worked example 4 — ice in sea water. Sea water is denser at about :
so about ** per cent is submerged and a little more shows. Denser liquid, less of the body under water — the same reason a person floats more easily in the sea than in a swimming pool.
Worked example 5 — wood.** A block of RD in water floats with ** per cent** submerged. Of a block, is under water.
Worked example 6 — half submerged. A body of density in a liquid of :
exactly half submerged.
Worked example 7 — working backwards. A block floats with of its volume submerged in water, so
The fraction never depends on the body's size. A small ice cube and an iceberg both float with per cent submerged, because cancelled out of the derivation. So "bigger things float lower" is false — bigger things displace more liquid and receive more upthrust in exact proportion, and the fraction is fixed by the two densities alone.
- — the body sinks
- — the body stays suspended, fully submerged but neither rising nor falling
- — the body floats, with part of it above the surface
Why the comparison decides it. Fully immerse a body of volume . Then
and the and are common to both, so whichever density is larger gives the larger force. **If the body is pushed up and rises; if it sinks; if they are equal it stays put.
The principle of floatation: a floating body displaces a weight of liquid exactly equal to its own weight. It sinks only far enough to find that much upthrust and no further.
Worked example 1 — classifying.** In water ():
- iron, — sinks
- ice, — floats
- cork, — floats
- a body of exactly — suspended
- mercury, — sinks
Worked example 2 — the same body in two liquids. A body of density :
- in water (): denser, so it sinks
- in a salt solution of : less dense, so it floats
One body, two outcomes — which is why does it float is an incomplete question until the liquid is named.
The fraction submerged. For a floating body, weight equals upthrust:
Worked example 3 — ice in water. , so ** per cent submerged** and per cent showing.
Worked example 4 — ice in sea water. Sea water is denser at about :
so about ** per cent is submerged and a little more shows. Denser liquid, less of the body under water — the same reason a person floats more easily in the sea than in a swimming pool.
Worked example 5 — wood.** A block of RD in water floats with ** per cent** submerged. Of a block, is under water.
Worked example 6 — half submerged. A body of density in a liquid of :
exactly half submerged.
Worked example 7 — working backwards. A block floats with of its volume submerged in water, so
The fraction never depends on the body's size. A small ice cube and an iceberg both float with per cent submerged, because cancelled out of the derivation. So "bigger things float lower" is false — bigger things displace more liquid and receive more upthrust in exact proportion, and the fraction is fixed by the two densities alone.
How do ships, submarines and hydrometers use floatation?
Each one manipulates one of the two densities in the ratio — the body's average density, or the liquid's.
A ship. Steel has a density of about and sinks in water as a solid block. A ship floats because it is hollow: the steel plus the enclosed air gives an average density well below .
Worked example 1. A ship of mass kg floating in water displaces
of water, so of its hull sits below the waterline. Loading cargo increases the mass, so the ship settles deeper until it displaces the extra weight — which is why load lines are painted on a hull.
A submarine. It changes its own average density by taking water into ballast tanks. Flooding the tanks raises its density above that of sea water and it dives; blowing the water out with compressed air lowers the density and it rises. Holding the density equal to the water's leaves it suspended at a chosen depth — the middle case of the previous section, used deliberately.
A hydrometer. A sealed glass float, weighted at the bottom so it stands upright, with a graduated stem. Its weight is fixed, so by the principle of floatation it always displaces the same weight of liquid — and therefore a smaller volume in a denser liquid.
So it sinks less deep in a denser liquid and deeper in a lighter one, and the stem reading gives the liquid's relative density directly. A lactometer is a hydrometer for milk, and one graduated for battery acid checks a car battery.
Worked example 2. A hydrometer of mass g floats in water and in a liquid of RD . In water it displaces
and in the denser liquid
Eight cubic centimetres less, so it rides higher — and that difference in depth is what the scale is calibrated against.
An iceberg. As calculated above, per cent is submerged in fresh water and about per cent in sea water. The hidden nine tenths is why icebergs are dangerous to ships — the visible peak gives no indication of the mass below it.
Floating in sea water against fresh water. A person of density close to floats with almost all of the body submerged in a freshwater pool, and noticeably higher in the sea at . Nothing about the swimmer changed — only the denominator of the ratio.
A ship rides lower in fresh water than in the sea. Moving from sea water into a river's fresh water reduces the liquid's density, so the ship must displace a greater volume to find the same weight of water, and it settles deeper. A ship loaded to its limit at sea can be overloaded the moment it enters a river, which is why load lines mark separate levels for salt and fresh water.
Hollowing a body changes its average density, not its material. That is the single idea behind the ship, and it is worth stating in exactly those words — the steel is as dense as ever, and the ship is not.
A ship. Steel has a density of about and sinks in water as a solid block. A ship floats because it is hollow: the steel plus the enclosed air gives an average density well below .
Worked example 1. A ship of mass kg floating in water displaces
of water, so of its hull sits below the waterline. Loading cargo increases the mass, so the ship settles deeper until it displaces the extra weight — which is why load lines are painted on a hull.
A submarine. It changes its own average density by taking water into ballast tanks. Flooding the tanks raises its density above that of sea water and it dives; blowing the water out with compressed air lowers the density and it rises. Holding the density equal to the water's leaves it suspended at a chosen depth — the middle case of the previous section, used deliberately.
A hydrometer. A sealed glass float, weighted at the bottom so it stands upright, with a graduated stem. Its weight is fixed, so by the principle of floatation it always displaces the same weight of liquid — and therefore a smaller volume in a denser liquid.
So it sinks less deep in a denser liquid and deeper in a lighter one, and the stem reading gives the liquid's relative density directly. A lactometer is a hydrometer for milk, and one graduated for battery acid checks a car battery.
Worked example 2. A hydrometer of mass g floats in water and in a liquid of RD . In water it displaces
and in the denser liquid
Eight cubic centimetres less, so it rides higher — and that difference in depth is what the scale is calibrated against.
An iceberg. As calculated above, per cent is submerged in fresh water and about per cent in sea water. The hidden nine tenths is why icebergs are dangerous to ships — the visible peak gives no indication of the mass below it.
Floating in sea water against fresh water. A person of density close to floats with almost all of the body submerged in a freshwater pool, and noticeably higher in the sea at . Nothing about the swimmer changed — only the denominator of the ratio.
A ship rides lower in fresh water than in the sea. Moving from sea water into a river's fresh water reduces the liquid's density, so the ship must displace a greater volume to find the same weight of water, and it settles deeper. A ship loaded to its limit at sea can be overloaded the moment it enters a river, which is why load lines mark separate levels for salt and fresh water.
Hollowing a body changes its average density, not its material. That is the single idea behind the ship, and it is worth stating in exactly those words — the steel is as dense as ever, and the ship is not.
Exam tip
Exam tip: relative density carries no unit, and hollow means average density
Relative density has NO unit. Write *RD *, never — that is the density.
**Convert with **, and remember .
**RD of a solid ** — air weight over the loss in water. gf and gf give .
**RD of a liquid ** — the loss in the liquid over the loss in water. , and gf give .
The unit of weight cancels in both formulas, so gf or N makes no difference.
The solid must SINK in both liquids for the liquid method to work.
Compare densities to decide: denser than the liquid sinks, equal stays suspended, less dense floats.
**Fraction submerged ** — ice in water gives , and in sea water .
The fraction does not depend on size — an ice cube and an iceberg both show a tenth.
For a floating body, weight equals upthrust, and it displaces its own weight of liquid.
For a ship or a hollow body, say AVERAGE density — the steel is unchanged; the hollow shape is what floats.
And for a hydrometer, say it displaces the same weight always, so a smaller volume in a denser liquid means it rides higher.
**Convert with **, and remember .
**RD of a solid ** — air weight over the loss in water. gf and gf give .
**RD of a liquid ** — the loss in the liquid over the loss in water. , and gf give .
The unit of weight cancels in both formulas, so gf or N makes no difference.
The solid must SINK in both liquids for the liquid method to work.
Compare densities to decide: denser than the liquid sinks, equal stays suspended, less dense floats.
**Fraction submerged ** — ice in water gives , and in sea water .
The fraction does not depend on size — an ice cube and an iceberg both show a tenth.
For a floating body, weight equals upthrust, and it displaces its own weight of liquid.
For a ship or a hollow body, say AVERAGE density — the steel is unchanged; the hollow shape is what floats.
And for a hydrometer, say it displaces the same weight always, so a smaller volume in a denser liquid means it rides higher.
Did you know
Why a ship sinks lower the moment it leaves the sea
A fully loaded cargo ship steams out of the ocean and up a river. Nothing is loaded or unloaded. The ship settles deeper in the water.
The reason is the principle of floatation, read carefully. A floating body displaces its own weight of liquid — not its own volume. The ship's weight has not changed, so the weight of water displaced cannot change either.
But fresh water is less dense than sea water, at about against . To find the same weight of a lighter liquid, the ship must push aside a larger volume — so more of the hull goes under.
Put numbers on it. A ship of mass kg displaces
in the sea, and
in the river — about more of hull below the waterline.
That is not a small matter for a ship loaded to its limit. A vessel safely laden at sea can become dangerously low in the water on entering a river, which is exactly why load lines painted on a hull mark separate levels for salt water and fresh water, and for different seasons.
The same effect works the other way for a swimmer. You float higher in the sea than in a freshwater pool, and you have not changed at all — the denominator of got bigger, so the submerged fraction got smaller.
And it is the whole working principle of a hydrometer, which turns the effect into a measurement: let something float, see how deep it sits, and read off the liquid's density.
The reason is the principle of floatation, read carefully. A floating body displaces its own weight of liquid — not its own volume. The ship's weight has not changed, so the weight of water displaced cannot change either.
But fresh water is less dense than sea water, at about against . To find the same weight of a lighter liquid, the ship must push aside a larger volume — so more of the hull goes under.
Put numbers on it. A ship of mass kg displaces
in the sea, and
in the river — about more of hull below the waterline.
That is not a small matter for a ship loaded to its limit. A vessel safely laden at sea can become dangerously low in the water on entering a river, which is exactly why load lines painted on a hull mark separate levels for salt water and fresh water, and for different seasons.
The same effect works the other way for a swimmer. You float higher in the sea than in a freshwater pool, and you have not changed at all — the denominator of got bigger, so the submerged fraction got smaller.
And it is the whole working principle of a hydrometer, which turns the effect into a measurement: let something float, see how deep it sits, and read off the liquid's density.
Exam relevance
How is floatation tested in JEE Main and NEET?
Because the density comparison decides every buoyancy problem, and the fraction-submerged result is the standard numerical of the topic.
This is the foundation for Class 11 Physics Mechanical Properties of Fluids, examined in JEE Main and NEET. The principle of floatation is used there in the same form, and the derivation on this page — setting equal to — is the standard proof of the submerged fraction. Numericals asking what fraction of a block floats above a liquid, or what density a body must have to float with a stated fraction under, are recurring in both exams.
The accelerating-frame version is a favourite extension. In a lift accelerating upward, both the weight and the upthrust scale by the same factor, since and share the . So the fraction submerged is completely unchanged — a result that feels wrong and is a standard assertion-reason item in JEE Main. The same reasoning says a floating body in a freely falling lift floats exactly as before.
Two-liquid problems extend the classification. A body may float at the interface between two immiscible liquids, partly in each, and the balance equation gains one more term — but every term is from this page.
Where relative density reappears. Class 11 Chemistry uses specific gravity for the same quantity, and the unitlessness matters there as well. NEET Biology uses floatation for the swim bladder of a fish, which works exactly like a submarine's ballast tank — changing the animal's average density to rise or sink — and for the flotation of seeds and organisms.
The ice-and-water numbers are worth remembering. That ice is less dense than its own liquid is unusual among substances, and it is why ice floats, why ponds freeze from the top, and why aquatic life survives a winter. NEET uses that chain in ecology questions, and the physics is the of this page.
Terminal velocity balances weight, upthrust and viscous drag, and the upthrust term is — so the numericals of the previous part of this chapter feed directly into it.
What the questions look like. For board work, expect define density and relative density with units, **convert between and , find RD of a solid and of a liquid from three weighings, state the principle of floatation, predict float, sink or suspended, calculate the fraction submerged, and explain a ship, a submarine, a hydrometer and an iceberg. For JEE Main and NEET, expect fraction-submerged numericals, floating in accelerating frames, two-liquid problems and terminal velocity.
How board and competitive emphasis differ. A board paper rewards the application explained in words — why a ship floats, how a hydrometer works — and the correct absence of a unit on an RD. A competitive paper assumes all of it and tests the density ratio in one step, often inside a larger force balance.
The single trap that costs the most marks.** Attaching a unit to a relative density, or using the full volume instead of the submerged volume in an upthrust. Ice in water has RD with no unit and floats with of its volume under, and writing for the RD or using the whole volume for the upthrust both give plausible wrong answers. The defence is to write the ratio with both densities visible — — so the units are seen to cancel and the submerged volume is what remains.
This is the foundation for Class 11 Physics Mechanical Properties of Fluids, examined in JEE Main and NEET. The principle of floatation is used there in the same form, and the derivation on this page — setting equal to — is the standard proof of the submerged fraction. Numericals asking what fraction of a block floats above a liquid, or what density a body must have to float with a stated fraction under, are recurring in both exams.
The accelerating-frame version is a favourite extension. In a lift accelerating upward, both the weight and the upthrust scale by the same factor, since and share the . So the fraction submerged is completely unchanged — a result that feels wrong and is a standard assertion-reason item in JEE Main. The same reasoning says a floating body in a freely falling lift floats exactly as before.
Two-liquid problems extend the classification. A body may float at the interface between two immiscible liquids, partly in each, and the balance equation gains one more term — but every term is from this page.
Where relative density reappears. Class 11 Chemistry uses specific gravity for the same quantity, and the unitlessness matters there as well. NEET Biology uses floatation for the swim bladder of a fish, which works exactly like a submarine's ballast tank — changing the animal's average density to rise or sink — and for the flotation of seeds and organisms.
The ice-and-water numbers are worth remembering. That ice is less dense than its own liquid is unusual among substances, and it is why ice floats, why ponds freeze from the top, and why aquatic life survives a winter. NEET uses that chain in ecology questions, and the physics is the of this page.
Terminal velocity balances weight, upthrust and viscous drag, and the upthrust term is — so the numericals of the previous part of this chapter feed directly into it.
What the questions look like. For board work, expect define density and relative density with units, **convert between and , find RD of a solid and of a liquid from three weighings, state the principle of floatation, predict float, sink or suspended, calculate the fraction submerged, and explain a ship, a submarine, a hydrometer and an iceberg. For JEE Main and NEET, expect fraction-submerged numericals, floating in accelerating frames, two-liquid problems and terminal velocity.
How board and competitive emphasis differ. A board paper rewards the application explained in words — why a ship floats, how a hydrometer works — and the correct absence of a unit on an RD. A competitive paper assumes all of it and tests the density ratio in one step, often inside a larger force balance.
The single trap that costs the most marks.** Attaching a unit to a relative density, or using the full volume instead of the submerged volume in an upthrust. Ice in water has RD with no unit and floats with of its volume under, and writing for the RD or using the whole volume for the upthrust both give plausible wrong answers. The defence is to write the ratio with both densities visible — — so the units are seen to cancel and the submerged volume is what remains.
Key takeaways
Density, relative density and the principle of floatation: quick revision
- Density , in or . Relative density and has NO unit.
- ****, since g kg and .
- Water , ice , cork , iron , mercury .
- A g, block has ; a piece of density has mass g.
- **RD is numerically the density in **, because water is exactly there — but the RD takes no unit.
- RD of a solid : and gf give ; and N give ; and gf give .
- RD of a liquid : , and gf give ; a reading of gf gives .
- The unit of weight cancels, and one set of three weighings gives both relative densities.
- The solid must sink in both liquids for the liquid method to work.
- Denser than the liquid sinks; equal stays suspended; less dense floats — because and share and .
- Principle of floatation: a floating body displaces a weight of liquid equal to its own weight.
- **Fraction submerged **, from .
- Ice in water: , so per cent under. In sea water (): , so a little more shows.
- Wood of RD floats per cent under — of a block. A body of in a liquid of floats exactly half under.
- A block floating submerged in water has .
- The fraction is independent of size — an ice cube and an iceberg both show a tenth.
- A ship floats because hollowing gives it a low average density; a kg ship displaces of fresh water.
- A submarine floods ballast tanks to dive and blows them with compressed air to rise, holding equal density to stay suspended.
- A hydrometer always displaces the same weight, so a smaller volume in a denser liquid — it rides higher. A g float displaces in water and in a liquid of RD .
- A ship rides lower in fresh water than in the sea, needing against — hence separate load lines.
- A swimmer floats higher in the sea for the same reason.
Float a small potato in plain water and then in water with plenty of salt stirred in, and see how much higher it sits — then work out the salt solution's density from the change.
- ****, since g kg and .
- Water , ice , cork , iron , mercury .
- A g, block has ; a piece of density has mass g.
- **RD is numerically the density in **, because water is exactly there — but the RD takes no unit.
- RD of a solid : and gf give ; and N give ; and gf give .
- RD of a liquid : , and gf give ; a reading of gf gives .
- The unit of weight cancels, and one set of three weighings gives both relative densities.
- The solid must sink in both liquids for the liquid method to work.
- Denser than the liquid sinks; equal stays suspended; less dense floats — because and share and .
- Principle of floatation: a floating body displaces a weight of liquid equal to its own weight.
- **Fraction submerged **, from .
- Ice in water: , so per cent under. In sea water (): , so a little more shows.
- Wood of RD floats per cent under — of a block. A body of in a liquid of floats exactly half under.
- A block floating submerged in water has .
- The fraction is independent of size — an ice cube and an iceberg both show a tenth.
- A ship floats because hollowing gives it a low average density; a kg ship displaces of fresh water.
- A submarine floods ballast tanks to dive and blows them with compressed air to rise, holding equal density to stay suspended.
- A hydrometer always displaces the same weight, so a smaller volume in a denser liquid — it rides higher. A g float displaces in water and in a liquid of RD .
- A ship rides lower in fresh water than in the sea, needing against — hence separate load lines.
- A swimmer floats higher in the sea for the same reason.
Float a small potato in plain water and then in water with plenty of salt stirred in, and see how much higher it sits — then work out the salt solution's density from the change.