Density (WJEC GCSE Science (Double Award): Physics): Revision Note

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:

$\mathrm{density}=\frac{\mathrm{mass}}{\mathrm{volume}}$

$\rho =\frac{m}{V}$

• Where: 

  • ρ = density measured in kilogram per metres cubed (kg/m³)

  • m = mass measured in kilograms (kg)

  • V = volume measured in metres cubed (m³)

• Gases are less dense than liquids because the particles in a gas are more spread out

• Liquids are less dense than solids, because the particles in a solid are closer together

Gases are less dense than solids because there are fewer particles per unit volume

• Students studying the Higher Tier will need to be able to rearrange the density equation

• The equation triangle helps with the rearrangement:

Equation Triangle for Density, Mass and Volume

Cover up the variable you need to calculate, and the triangle shows you how the equation is arranged

• 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

Volumes of common 3D shapes