AQA GCSE Physics

Revision Notes

3.1.1 Density

Density

  • Density is defined as:

The mass per unit volume of a material

  • Objects made from low density materials typically have a low mass
  • Similarly sized objects made from high density materials have a high mass
    • For example, a bag full of feathers is far lighter compared to a similar bag full of metal
    • Or another example, a balloon is less dense than a small bar of lead despite occupying a larger volume
  • Density is related to mass and volume by the following equation:

Density equation, downloadable AS & A Level Physics revision notes

  • Gases, for examples, are less dense than solids because the molecules are more spread out (same mass, over a larger volume)

Density comparison, downloadable AS & A Level Physics revision notes

Gases are less dense than solids

  • This equation can be rearranged with the help of the formula triangle:

Density triangle (2), IGCSE & GCSE Physics revision notes

Density, mass, volume formula triangle

  • The units of density depend on what units are used for mass and volume:
    • If the mass is measured in g and volume in cm3, then the density will be in g/cm3
    • If the mass is measured in kg and volume in m3, then the density will be in kg/m3
  • This table gives some examples of densities on common materials
    • If a material is more dense than water (1000 kg/m3), then it will sink

Approximate Densities of Materials Table

Table of densities of common materials, downloadable IGCSE & GCSE Physics revision notes

  • The volume of an object may not always be given directly, but can be calculated with the appropriate equation depending on the object’s shape

Volume of shapes, downloadable AS & A Level Physics revision notes

Volumes of common 3D shapes

Worked Example

A paving slab has a mass of 73 kg and dimensions 0.04 m × 0.5 m × 0.85 m.

Worked Example Density, downloadable IGCSE & GCSE Physics revision notes

Calculate the density, in kg/m3, of the material from which the paving slab is made.

Step 1: List the known quantities

    • Mass of slab, m = 73 kg
    • Volume of slab, V = 0.04 m × 0.5 m × 0.85 m = 0.017 m3

Step 2: Write out the equation for density

Step 3: Substitute in values

ρ = 73 ÷ 0.017 = 4294 kg/m3

Step 4: Round the answer to two significant figures

ρ = 4300 kg/m3

Exam Tip

Make sure you are comfortable converting between units such as metres (m) and centimetres (cm) or grams (g) and kilograms (kg).

  • When converting a larger unit to a smaller one, you multiply (×)
    • E.g. 125 m = 125 × 100 = 12 500 cm
  • When you convert a smaller unit to a larger one, you divide (÷)
    • E.g. 5 g = 5 ÷ 1000 = 0.005 or 5 × 10-3 kg

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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