Density (OCR AS Physics): Revision Note
Exam code: H156
Density
Density is the mass per unit volume of an object
Objects made from low-density materials typically have a lower mass
For example, a balloon is less dense than a small bar of lead despite occupying a larger volume
The units of density depend on the units 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
Gases are less dense than a solid
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
Worked Example
A paving slab has a mass of 73 kg and dimensions 40 mm × 500 mm × 850 mm.
Calculate the density, in kg m-3 of the material from which the paving slab is made.
Answer:
Examiner Tips and Tricks
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
When dealing with squared or cubic conversions, cube or square the conversion factor too
E.g. 1 mm3 = 1 / (1000)3 = 1 × 10-9 m3
E.g. 1 cm3 = 1 / (100)3 = 1 × 10-6 m3
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