Mouhefanggai
This article derives the formula for the volume of a sphere, without using any mathematics more advanced than Pythagoras’ theorem. It uses some ideas developed in the article Volume of a Pyramid and a Cone, which you may want to read first.
Volume of a mouhefanggai
Imagine two identical cylindrical pipes meeting at right angles. Now think about the shape of the space which belongs to both pipes. This is the shape that early Chinese mathematicians called the mouhefanggai. This can be translated as “two square umbrellas”.
Making a mouhefanggai is tricky. You can slice two marrows so that they fit together like the pipes in the diagram. The bits you’ve sliced out can be put together to form a mouhefanggai. Alternatively, if you can find a couple of the square net umbrellas used to protect cakes from flies, this will make a reasonable approximation.
It might seem unlikely, but you can find a formula for the volume of a mouhefanggai using relatively elementary maths. This article will show you how to do this, and then go on to find the volume of a sphere, a formula which is often quoted but rarely proved without using calculus.
Observations about the mouhefanggai
Let’s suppose that the cylinders have radius r, and that they are both lying on a horizontal plane. Then the mouhefanggai where they intersect has height 2r. If we slice the mouhefanggai horizontally, each slice will be a square. The largest square, in the middle, will have sides length 2r.
We can actually simplify things slightly, by dealing with just part of the mouhefanggai. We can cut it into eight identical pieces, by slicing it in three perpendicular directions. The horizontal cut gives a ‘tent shaped’ solid with a flat base and four curved ‘walls’. This is cut into four with two perpendicular vertical cuts.
The difference solid
Take one of our pieces of the mouhefanggai, and imagine it fitting into a cube of side r, as shown:
Rather than finding the volume of the mouhefanggai piece, we shall actually find the volume of the rest of the cube; the difference-solid between the cube and the mouhefanggai piece.
Taking a slice
Imagine taking a horizontal slice across the cube, height h above the base. If we look down at this slice, we will see a square side r. Of this, a square in one corner is part of the mouhefanggai, and the remaining L-shape is part of the difference-solid. We’ll suppose, for now, that the mouhefanggai square has side-length k. So the area of the L-shape will be r2−k2.
Now let’s think again where that slice came from. If you look at this front view of the cube, you will see that h, k and r form a right-angled triangle. So using Pythagoras’ Theorem, r2=h2+k2. Rearranging this, we get r2−h2=k2. So the area of the L-shape is r2−k2=h2. Can you think of another solid, which when we take a slice at height h gives us a slice of area h2? In other words, the bottom slice has zero area, and the area gradually increases, to a top slice of area r2.
When you have thought about this, you can look here for a picture of a solid which has slices with the same area as our difference-solid.
Well, we know how to find the volume of the shape on the left, and if all the slices are the same, the difference-solid must also have volume r33.
Back to the mouhefanggai
If the cube has volume r3 , and the difference-solid has volume r33, then the mouhefanggai piece has volume 23r3. So the whole mouhefanggai has volume 163r3. Phew!
Comparing a sphere with a mouhefanggai
Although it surprised me that we could find the volume of a mouhefanggai like that, it’s not a shape I can imagine needing to work with often. A sphere, though, is another matter. Most people have to just believe what they are told about the volume of a sphere until they have learnt calculus. But having found the volume of a mouhefanggai, it is a relatively small step to find the volume of a sphere.
Think back to the previous article, where we found the volume of a cone by comparing it with the square-based pyramid it fitted into.
Now picture a sphere fitting snugly inside a mouhefanggai!
Any horizontal slice will consist of a circle radius x inside a square of side 2x. The area of the circle is πx2. The area of the square is 2x×2x=4×2. The ratio of the circle to the square is π:4. So, for each slice, the area of the circle is π4 the size of the area of the square. This means that the volume of the sphere will be π4 the volume of the mouhefanggai. Volume of a sphere=π4×163r3=43πr3.
One to try yourself: doughnut
Imagine a perfect doughnut, and compare it to a cylinder:
Slices at height x will look like this:
Compare the slices, and hence compare the volumes of the two solids.