ACARA v9 CONTENT DESCRIPTION “solve problems involving the surface area and volume of composite objects using appropriate units”
Builds on: Surface Area and Volume of Right Prisms and Cylinders (AC9M9M01). The formulas for the surface area and volume of single solids are the tools this unit assembles. Here those known solids are combined into composite objects, where the new skill is keeping careful track of what to add and what to leave out.
Building shapes from shapes
Real objects are rarely a single neat box or cylinder. A house with a triangular roof, a bottle, a bench with a hollow, a machine part with a hole drilled through it: each is a composite object, made by joining simple solids together or by cutting a piece away. The good news is that you do not need any new formulas. A composite object is handled by breaking it back into the simple solids you already know, a box, a cylinder, a cone, a sphere, working out the volume and surface area of each, and then combining those results with care. The whole skill of this unit lies in that final combining step, because volume and surface area combine in two quite different ways.
Composite objects are simple solids joined
A composite object is made by combining solids whose volume and surface area you already know.
A composite object is built from simple solids such as boxes, cylinders, cones and spheres. Here are two boxes. Join them to see the composite shape this unit is about.
Volume is the straightforward one
Volume behaves exactly as intuition expects: it adds. The space taken up by a composite object is simply the space taken up by all its parts together, so you find the volume of each simple solid and add them. A small box of eight cubic units sitting on a larger box of forty cubic units makes a composite of forty-eight cubic units, full stop. Whether the parts touch, and where they touch, makes no difference to volume, because filling space is filling space. When a piece is removed instead of added, as with a hole, you subtract that volume. So for volume the rule is short: add the parts you join, subtract the parts you remove, and the touching faces never enter into it.
Volume simply adds up
The volume of a composite object is the sum of the volumes of the simple solids that make it.
For a composite object, the volume is the sum of the volumes of its parts. Reveal the part volumes to add them up.
Surface area needs more care
Surface area is where the thinking really happens, because it does not simply add. Surface area measures only the outside of an object, and when two solids are joined, the faces where they meet are pressed together and tucked away inside, no longer part of the outside at all. If you just added the two full surface areas, you would be counting those hidden faces, which are not on the surface anymore. The correct method is to add the two surface areas and then subtract the area of contact twice, once because it disappears from the first solid and once because it disappears from the second. Forgetting to remove the hidden faces is the single most common error in these problems, so it is worth picturing exactly which faces vanish before you start calculating.
Surface area: the joined face is hidden
When solids join, the touching faces are no longer on the outside, so they are subtracted from the total surface area.
Surface area does not simply add, because the faces where the solids join are hidden inside. Reveal the contact face to see what must be left out.
When a piece is taken away
Composite objects are not always built by adding; many are made by removing a piece, and removal needs the same careful bookkeeping. Drill a cylindrical hole straight through a block and the volume clearly drops by the volume of that cylinder. The surface area, though, changes in two directions at once. The two circular faces where the hole opens are punched out of the block, removing a little surface, but the curved inner wall of the tunnel is now exposed, adding a good deal more. So a hole can actually increase the total surface area even as it decreases the volume. The principle for every composite, joined or cut, is the same: track each face honestly, asking whether it ends up on the outside or not.
Removing a piece: holes and hollows
A composite can be formed by subtracting a solid, which removes volume but may add new inner surface.
Not all composites are built by joining; some are made by cutting a piece away. Bore a hole through this block to see how removal changes volume and surface area differently.
Getting the units right
A final discipline ties the topic together: using appropriate units. Surface area, because it counts how many unit squares would cover the outside, is always measured in square units, such as square centimetres or square metres. Volume, because it counts how many unit cubes would fill the inside, is measured in cubic units, such as cubic centimetres or cubic metres. Mixing these up, reporting a volume in square units or an area in cubic units, turns a correct calculation into a meaningless answer, and in real contexts, ordering paint by the cubic metre or concrete by the square metre, it is an expensive mistake. Always pause at the end to check that the unit matches the quantity: squares for surface, cubes for volume.
Appropriate units: square versus cubic
Surface area uses square units and volume uses cubic units; matching the unit to the quantity is essential.
Units must match the quantity. Surface area is measured in square units, such as square metres, because it counts how many unit squares cover the outside. Switch to volume to compare.
Quick self-check
1. A composite object is:
2. The volume of a composite made by joining two solids is:
3. When two solids are joined, the surface area of the composite is:
4. A cylindrical hole is bored straight through a solid block. The surface area:
5. Surface area and volume are correctly measured in: