90
$\begingroup$

The following shape:

unwrapped cube

can be folded onto the surface of a cube in a way that perfectly covers the entire cube with no gaps and no overlaps.

How can this be done?

$\endgroup$
147
$\begingroup$

The shape

has an area of 30, if each square is taken to be 1 unit. So one face of the cube must have area 5: the easiest way to make an area-5 square on a lattice is by using a knight's move as your side.

Using this, we can make a guess for how the cube might be folded:

cube folding, part 1
Overlaying this net looks nice: the top of the cube is nearly done, the four sides around it look pretty good, and the back is mostly uncovered. The black shapes are good, but the holes in the middle are a problem. We can solve this problem by taking the gray sections along the border, and folding them along the lines between the four sides, as shown by the blue arrows. (Remember, the four sides really connect to each other! The blue arrows are legal folds, even though they might not seem like it at first.)

Once that fold is done, the shape looks more like this:

enter image description here
This is the same shape in 3D, and it's still connected as it was before, but I've moved some of the pieces across the seams of the net. Now it should be pretty clear how the rest folds up: each piece still hanging off the cube will be folded to cover 1/4 of the bottom face (as the lower right piece already shows), and then wrapped around to fill the small triangular hole in a different side of the cube.

A drawing of the finished product:

enter image description here
Here, one "arm" of the original shape has been colored dark gray to show how it folds up.

And an animation of the whole process:

enter image description here

$\endgroup$
  • 41
    $\begingroup$ +1, nice diagrams $\endgroup$ – micsthepick Jun 12 at 7:54
  • 19
    $\begingroup$ Let's say I knew the answer right off the bat (I didn't). There's no way I could construct an answer like this within 1 hour of the question being asked. How much of that CPH4 did you actually take? $\endgroup$ – Strawberry Jun 12 at 16:32
  • 4
    $\begingroup$ Impressive. This is a much better explanation than I could have come up with in twice the time. (and you presumably had to spend some of that time actually solving the problem!) $\endgroup$ – plasticinsect Jun 12 at 16:44
  • 11
    $\begingroup$ @Wossname Whoa, thanks for adding that animation! That's really cool (and I wish there was some way to give you a bounty for it, because you deserve credit for it). $\endgroup$ – Deusovi Jun 14 at 6:22
  • 8
    $\begingroup$ @Wossname Wow, that animation is beautiful. May I ask what software you did it on? $\endgroup$ – plasticinsect Jun 14 at 7:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.