Why does the Euler's formula work on a cylinder?

Answer 1

I suppose you mean the formula V + F - E = 2. A simple example is a cube, which has 8 vertex points, 6 faces, and 12 edges, so 8 + 6 - 12 = 14 - 12 = 2. The faces of a cube are flat, but this would also work if the faces or edges were somewhat curved, just so long as they don't intersect each other. The reason I mention this is that in the case of a cylinder, one of the faces will be the curved part of the cylinder. Let's concentrate first on that curved face, because that is the "secret" about interpreting Euler's Law for a cylinder. If you examine a 'tin can', common as a food container, you will see that there is a 'seam' from bottom to top. That seam counts as an edge for the purposes of the formula. Also, that edge has a vertex at each of its ends. When we close up the cylinder 'tin can', the top is a circular face, with an edge around its circumference, and the same for the bottom face. Here is a list of all the faces, edges and vertices. Face 1 = the curved surface around the cylinder. Face 2 = the top, which is flat Face 3 = the bottom, which is also flat Edge 1 = the seam up the side of the curved face Edge 2 = the circle around the top face Edge 3 = the circle around the bottom face. Vertex 1 = the point at the top of the seam Vertex 2 = the point at the bottom of the seam Note that the two vertices also serve double-duty as the points where the circular edges start and stop. So, V + F - E = 2 + 3 - 3 = 2 + 0 = 2 This is not the only way to make a 'tin can'. You could do it with two seams from bottom to top. Think of it as making two half-cans, the front half and the back half, and then putting them together along the seams to enclose the cylindrical volume. How does this change things? For one, there is an extra seam, which means an additional edge. But that's not all. The extra seam has a vertex at each end, and that breaks the top and bottom circle edges into two parts each. So E = 6 instead of 3. The curved face which previously went all around is now replaced by two faces, each of which goes half way around, so F = 4 instead of 3. There are two more vertices, namely, the points at the ends of the new seam edge, so V = 4 instead of 2. So V + F - E = 4 + 4 - 6 = 2 The Law is verified under this configuration, too.