Infinite Wrapping Paper

Calculus Level 1

It's nearly Christmas, and you are shopping for presents for your family. Each present you buy must have a volume of V 1 m 3 V\leq1 \text{ m}^3 , and may have any shape at all. Is it possible to buy a present that requires an infinite amount of wrapping paper?

Don't worry about fitting the present under the tree.

More information needed Yes No

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2 solutions

Fin Moorhouse
Dec 15, 2015

There are many examples of shapes with infinite surface area and finite volume- let's find an example. One contender may be the appropriately festive 'Koch Snowflake':

This fractal shape has a finite area, but an infinite perimeter, so could a 3D Koch snowflake, or something similar, be the shape we are after? No, because the question does not specify that the wrapping paper has to precisely follow the edge of the shape, so we could fit such a shape into a sphere of wrapping paper, for instance. Let's try another example, which we will call a 'magic staircase'. It has steps of equal length and width, but the height of each step is half that of the previous step. The first few steps may look something like this:

Recall that 1 2 + 1 4 + 1 8 + 1 16 . . . = 1 \frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}...=1 . Therefore, if the height of the first step (from the ground) is 0.5 m 0.5m and the distance between steps is 1 m 1m , then the area of the front-facing side of the staircase will be 1 m 2 1m^2 . Let's take the width of the stairs to be 1 m 1m , making the volume of our magic staircase 1 m 3 1m^3 . However, it is intuitively clear that such a shape will require an infinite amount of wrapping paper, since the staircase will stretch on forever in the horizontal direction (the shape's surface area is infinite). Therefore the answer is yes, you can buy a present with finite volume requiring an infinite amount of wrapping paper. A more elegant example of such a shape is called Gabriel's Horn .

Wouldn't the perimeter of a fractal begin to approach a limit? The stairs example is great though.

Josh Mcdevitt-Spall - 5 years, 5 months ago

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Yes, and the limit in a lot of cases is \infty - see this link for more (and thanks!).

Fin Moorhouse - 5 years, 5 months ago

Could the perimeter of the snowflake converge to finite value as there seems to be no infinite length to occupy the space? Gabriel's Horn could only be a mathematical concept as any wrapping papers of infinite area must not be finite volume due to the fact of building blocks of atoms. The earth cannot accommodate infinite area of infinite length and no one could sell or buy such thing.

Lu Chee Ket - 5 years, 4 months ago

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Fair point, but when have the contingencies of the real world ever got in the way of maths?

Fin Moorhouse - 5 years, 4 months ago

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I appreciate the fact of something which is not naturally easy able to think about. But the point is wrapping paper you described ought to relate to atoms by which the question could not be answered with no ambiguity and no doubt. The infinite length of Gabriel's Horn cannot persist as it has arrived to diameter of an atom. Consequently, only finite area and finite volume for paint can be there.

Lu Chee Ket - 5 years, 4 months ago
Rarity SoChic
Dec 23, 2015

A sphere is an example of shapes with infinite surface but finite volume e.e

I'm afraid it isn't. The surface area of a sphere is 4 π r 2 4\pi r^2 .

Fin Moorhouse - 5 years, 5 months ago

I think you must reconsider whether you have written what you intended.

Manish Mayank - 5 years, 5 months ago

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