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7.11Physics 01. Forces And Energy

Use density to explain why objects float or sink in water — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions

5 min read 10 parts Lesson 12 of 12 1 quiz

📚 Cambridge Checkpoint Science 0893 Hub / Topical Revision
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7.11 Density, floating and sinking — CHECKPOINT - Lower Secondary Science 0893 notes: Use density to explain why objects float or sink in water.Density — the mass of a substance in a given volume.Equation: density = mass ÷ volume.Units — g/cm3 or kg/m3.Denser than water — an obje…

  • Less dense than water
  • Density of water
  • Hollow shape
  • Average density
  • Oil spills
  • Helium balloon

Section 1 of 10Density

Density
  • Density the mass of a substance in a given volume.
  • Equationdensity = mass ÷ volume.
Units
  • Units g/cm3 or kg/m3.

Section 2 of 10Floating and sinking in water

Denser than water
  • Denser than water an object with a density greater than that of water sinks.
Less dense than water
  • Less dense than water an object with a density less than that of water floats.
Density of water
  • 1.0 g/cm3, or 1000 kg/m3.

Section 3 of 10Densities of materials

MaterialDensity in g/cm3In water
Soft wood0.35 to 0.52Floats
Hard wood0.60 to 0.95Floats
Cooking oil0.91 to 0.93Floats on water
Water1.0Reference
Concrete2.4Sinks
Aluminium2.7Sinks
Iron7.9Sinks

Section 4 of 10Finding density to predict floating

1
Measure the mass of the object with a balance.
2
Find the volume by measuring the sides of a regular object or by displacement of water for an irregular object.
3
Divide the mass by the volume.
4
Compare the result with 1.0 g/cm3.

Section 5 of 10Why steel ships float

Steel
  • Steel the density is 8 to 9 g/cm3, much more than water.
Hollow shape
  • Hollow shape a ship contains large air spaces, so its total volume is very large.
Average density
  • Average density the mass of the whole ship including the air divided by the total volume is less than the density of water.
Cargo
  • Cargo loading cargo, fuel and passengers adds mass and raises the average density, and a ship can sink if its average density exceeds that of water.

Section 6 of 10Objects designed to float

  • Swimming floats are made of polystyrene or are hollow with air inside.

Section 7 of 10Liquids of different density

Layers
  • Layers less dense liquids float on denser liquids.
  • Vegetable oil floats on water, which floats on corn syrup.
Oil spills
  • Oil spills crude oil is less dense than water, so it floats on the sea and washes up on beaches.

Section 8 of 10Gases and density

Helium balloon
  • Helium balloon it floats in air because helium is less dense than air.
Hot air balloon
  • Hot air balloon the heated air expands and becomes less dense than the cooler air outside.
Bubbles in drinks
  • Bubbles in drinks carbon dioxide gas is less dense than the liquid so the bubbles rise.

Section 9 of 10Key terms

TermMeaning
DensityMass per unit volume.
Average densityTotal mass divided by total volume, including any air spaces.
HollowHaving a space inside, often filled with air.
DisplacementMoving aside of water by an object, used to find volume.
FloatStay on the surface of a liquid because the density is lower.

Section 10 of 10Exam technique

1
Large objects sink and small ones float — density decides, not size.
2
Heavy objects always sink — a heavy ship floats because its average density is low.
3
Density changes if the object is cut — the density stays the same.
4
Floating means no force acts — the weight is balanced by the upward force from the water.

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