Excursion: Separating substances with a nail board?

Complex Sorting Made Easy

Imagine you have a box full of different marbles that you absolutely have to sort: You can pick out the marbles one by one and put them into different boxes individually. But that’s quite complex. Admittedly, for a small number of marbles as shown here, that’s certainly a good solution, but the more marbles you have to sort, the more complex it becomes.

There must be a better way, don’t you think? If we make use of how the different types of marbles differ from each other, we can enormously speed up the process: In our example, we want to separate two different types of marbles: The purple marbles are large and made of styrofoam, so they are very light. The turquoise marbles are small and made of metal, and therefore heavier.

If we pour the contents of our box through a special nail board apparatus, as shown in the following picture series, the marbles will be separated as if by magic. We only have to replace the box under the apparatus with an empty one once, and we have separated the purple and turquoise marbles.

(Tip: Select the full-screen option in the top right of the picture series, then you’ll see best what’s happening.)

And why does it work?

The secret behind this method lies in the different properties of the marbles: they fall through the apparatus at different speeds. You can imagine it like this: Each marble, on its way through the board, repeatedly hits the nails and is thus held up; depending on how heavy and how large the marble is, it is slowed down to different degrees.

In general: Large marbles are held up more than small ones, and light marbles more than heavy ones. That’s why, in our example, the turquoise marbles fall through the apparatus significantly faster than the purple ones, because they are both smaller and heavier.