# Sparks and Seeds for Learning

We often talk about ideas being sparked and laying seeds for inspiration, so here is a collection of resources, seeds, and sparks for us to share. Who knows what exciting lessons, work, and discussions could come from this!

## Sunday, 16 September 2018

## Wednesday, 25 July 2018

### Cubes from all Angles

Labels:
Geometry,
Maths,
Transformations

### Roller Coaster Physics

## Roller Coaster Physics.

The principle that underlies the working of a roller coaster is simple. As you ascent to the top, your potential energy builds up. ( i.e the higher you go, longer the distance that the force of gravity can act upon)

This built up Potential energy gets released as kinetic energy downhill. ( Kinetic energy is the energy of motion- linear and rotational).

Now what Roller Coaster Engineers do is abuse this principle to engineering perfection.

### Fly Mobius Strip

Mobius Fly Maze

Courtesy of David Phillips

"Find the path that the four flies take if they all travel the same route without meeting, and without retracing their path until they reach their original position. Keep track of which side of the path you are on."

Courtesy of David Phillips

"Find the path that the four flies take if they all travel the same route without meeting, and without retracing their path until they reach their original position. Keep track of which side of the path you are on."

This is an example of a Mobius Strip, where a surface has only one side and one boundary. It is mathematically 'unorientable'.

### Can You Flatten a Sphere?

## Can you flatten a sphere?

The answer is

**NO**, you can not. This is why all map projections are innacurate and distorted, requiring some form of compromise between how accurate the angles, distances and areas in a globe are represented.
This is all due to Gauss’s

*Theorema Egregium*, which dictates that you can only bend surfaces without distortion/stretching if you don’t change their*Gaussian curvature*.
The Gaussian curvature is an intrinsic and important property of a surface. Planes, cylinders and cones all have zero Gaussian curvature, and this is why you can make a tube or a party hat out of a flat piece of paper. A sphere has a positive Gaussian curvature, and a saddle shape has a negative one, so you cannot make those starting out with something flat.

If you like pizza then you are probably intimately familiar with this theorem. That universal trick of bending a pizza slice so it stiffens up is a direct result of the theorem, as the bend forces the other direction to stay flat as to maintain zero Gaussian curvature on the slice. Here’s a Numberphile video explaining it in more detail.

However, there are several ways to

*approximate*a sphere as a collection of shapes you can flatten. For instance, you can project the surface of the sphere onto an icosahedron, a solid with 20 equal triangular faces, giving you what it is called the Dymaxion projection.*The Dymaxion map projection.*

The problem with this technique is that you still have a sphere approximated by flat shapes, and not curved ones.

One of the earliest proofs of the surface area of the sphere (4πr

^{2}) came from the great Greek mathematician Archimedes. He realized that he could approximate the surface of the sphere arbitrarily close by stacks of*truncated cones*. The animation below shows this construction.
The great thing about cones is that not only they are curved surfaces, they also have zero curvature! This means we can flatten each of those conical strips onto a flat sheet of paper, which will then be a good approximation of a sphere.

So what does this flattened sphere approximated by conical strips look like? Check the image below.

But this is not the only way to distribute the strips. We could also align them by a corner, like this:

All of this is not exactly new, of course. In the limit, what you have is called a American polyconic projection, which does require stretching in order to fill the gaps between the ending of the strips. Gauss’s

*Theorema Egregium*demands this.
But I never saw anyone assembling one of these polyconic approximations. I wanted to try it out with paper, and that photo above is the result.

It’s really hard to put together and it doesn’t hold itself up too well, but it’s a nice little reminder that

*after all!***math works**
Subscribe to:
Posts (Atom)