Modelling with Volumes of Revolution (Edexcel International A Level Maths: Pure 4)

What is meant by modelling with volumes of revolution? 

• Many every day objects such as buckets, beakers, vases and lamp shades can be modelled as a solid of revolution
• This can then be used to find the volume of the solid (volume of revolution) and/or other information about the solid that could be useful before an object is manufactured
• An object that would usually stand upright may have to be modelled horizontally so its volume of revolution can be found

An upright lamp shade modelled horizontally as a straight-line segment
( between  and )  rotated  around the -axis

What modelling assumptions are there with volumes of revolution? 

• The solids formed are usually the main shape of the body of the object
  • For example, the handles on a vase would not be included
  • The lip on the top edge of a bucket would not be considered
• A common question or assumption concerns the thickness of a container
  • The thickness is generally ignored as it is relatively small compared to the size of the object
    • thickness will depend on the purpose of the object and the material it is made from
  • Some questions may refer to the solid formed being the ‘inside’ of an object or refer to the ‘internal’ dimensions
  • If the thickness of the material is significant it would involve two related solids of revolution (Adding & Subtracting Volumes)

How do I solve modelling problems with volumes of revolution? 

• Visualising and sketching the solid formed can help with starting problems
• Familiarity with applying the volume of revolution formulae for rotations around both the x-axis

x-axis 

• The volume of a solid may involve adding or subtracting different volumes of revolution
  • Subtraction would need to be used for solids formed from areas that do not have a boundary with the axis of rotation
• Questions may go on to ask related questions in context so do take notice of the context
  • A question about a bucket being formed may ask about its capacity
• This would be measured in litres so there may also be a mix of units that will need conversion (e.g. 1000cm³ = 1 litre)