1.3.4Motion, Forces and Energy
Floating — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 24 of 91 1 quiz
Lesson summary
1.3.4 Floating — IGCSE PHYSICS notes: Use densities to decide whether an object floats or sinks, and explain why steel ships float.Less dense than the fluid — the object floats.More dense than the fluid — the object sinks.Equal densities — the object stays at any depth (neutral b…
- Typical blocks in water
- Average density
- Plasticine
- Fresh water and salt water
- Floating liquids (Extended)
- Hot-air balloon
Part 1 of 7Density rule for floating
Density rule for floating
Less dense than the fluid
- Less dense than the fluid the object floats.
More dense than the fluid
- More dense than the fluid the object sinks.
Equal densities
- Equal densities the object stays at any depth.
- (neutral buoyancy)
Floating always refers to one object in one liquid.
- Cork (240 kg/m3) floats on water (1000 kg/m3) but iron (7900 kg/m3) sinks.
- Iron floats on mercury (13 600 kg/m3) because it is less dense than mercury.
Typical blocks in water
- Typical blocks in water polystyrene (0.05 g/cm3) floats, wood (0.9 g/cm3) floats partly submerged and iron (7.9 g/cm3) sinks.
Part 2 of 7Upthrust
Upthrust
Upthrust the upward force a fluid exerts on an object placed in it.
Cause
- Cause the pressure at the bottom of the object is greater than the pressure at the top.
Direction
- Direction upthrust acts opposite to the weight.
Size of upthrust depends on:
- the density of the fluid, and
- the volume of fluid displaced, which equals the volume of the object that is submerged.
Upthrust equals the weight of the fluid displaced by the object.
Part 3 of 7Floating and sinking in terms of forces
Floating
- Floating upthrust equals weight, so the forces are balanced and the resultant force is zero.
Sinking
- Sinking upthrust is less than weight, so the resultant force is downwards and the object accelerates downwards.
A denser object can never displace enough fluid to produce an upthrust equal to its weight.
A floating object has weight, and it sinks until the weight of fluid displaced equals its own weight.
Part 4 of 7Why a steel ship floats
Steel is nearly eight times denser than water, but a ship is a hollow shell filled mainly with air.
Average density
- Average density the mass of the ship divided by its total volume, including the air, is less than the density of water.
The hull sinks until the weight of water displaced equals the weight of the ship.
A loaded ship sits lower in the water than an empty one; the load line on the hull shows the safe depth.
Plasticine
- Plasticine a lump sinks but a boat-shaped piece floats because it displaces much more water for the same mass.
Part 5 of 7Different liquids
Different liquids
Fresh water and salt water
- Fresh water and salt water a ship floats lower in fresh water than in salt water (about 1030 kg/m3) because it must displace more of the less dense liquid.
An egg sinks in fresh water but floats in a strong salt solution.
Floating liquids (Extended)
- Floating liquids (Extended) a liquid of lower density floats on a liquid of higher density if the two do not mix.
- Oil (about 800–900 kg/m3) floats on water because it is less dense.
- Unmixed liquids form layers with the densest at the bottom.
Part 6 of 7Applications
1Hot-air balloon
- Hot-air balloon the heated air inside has a lower density, so the average density of balloon, basket and air is less than that of the surrounding air.
2Submarine
- Submarine it changes its average density by pumping water into or out of ballast tanks.
3Fish
- Fish they adjust the gas in their swim bladders to match the density of the water at their depth.
4Fraction submerged (Extended)
- Fraction submerged (Extended) a floating object with average density 800 kg/m3 in water has 0.80 of its volume below the surface.
Part 7 of 7More from the specification
More from the specification
Mass, Weight, and Gravitational Field Strength
- Mass is the amount of matter in an object,
- (measured in kilograms using a digital balance)
- while weight is the downward gravitational force acting on it near Earth (W = m·g), proportional to gravitational field strength g (≈ 9.8 N/kg).
Density Determination of Regular and Irregular Objects
- The density of a solid cylinder is found by measuring its diameter and length with calipers to calculate volume (V = π·r²·L),
- while irregular solids are measured by liquid displacement in a measuring cylinder.
Upthrust and the Principle of Flotation
- An object floats in a fluid when upward buoyant force (upthrust) equals its total weight;
- floating occurs when the object's average density is less than or equal to the density of the surrounding liquid.
Length
- Resistance is directly proportional to the length of a wire.
Apparatus
- The equipment needed to carry out an experiment.
Variable
- A factor that can change in an investigation.
Gravitational field
- Region where mass experiences gravitational force.
Field strength
- G = Fg/m.
Vernier callipers
- An instrument measuring small lengths to 0.1 mm.
Independent variable
- The variable deliberately changed, here time
Dependent variable
- The variable measured, here temperature
Key terms
| Term | Meaning |
|---|---|
| Upthrust | Upward force from a fluid on an object in it. |
| Displaced liquid | Liquid pushed aside by an object. |
| Average density | Total mass divided by total volume, including hollow spaces. |
| Floating | Resting with weight equal to upthrust. |
| Neutral buoyancy | State where the density of the object equals that of the fluid. |
Avoid these mistakes
1
Saying heavy objects sink and light objects float — it is the density compared with the liquid that matters.
2
Saying steel ships float because steel is light — they float because their average density, including the air, is less than the water.
3
Believing a floating object has no weight — it has weight that is balanced by the upthrust.
4
Saying upthrust equals the volume of water displaced — it equals the weight of water displaced.
Marked practiceQuizzes for this lesson
Last updated:
\n