Density (OCR GCSE Physics A (Gateway)): 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

Calculating Density

• Density is related to mass and volume by the following equation:

• Where: 

  • ρ = density (kg/m³)

  • m = mass (kg)

  • V = volume (m³)

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

Gases are less dense than solids

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

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

• 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