The Secret Physics of Mixed Nuts

If you want to properly mix cashews and M&M’s, there’s an easy way and a hard way to do it. The easy way is to put in the cashews, then put in the M&M’s, and then shake it. The hard way is to try to stir them. And if you put the M&M’s in first, shaking it won’t work at all!

This is actually a perfect illustration of a useful physics trick called the Brazil Nut Effect — named after the way that, when you open a container of mixed nuts, the Brazil nuts (which almost nobody likes) always seem to be on top. The general rule is that the biggest items will always end up on top when you shake a mixture.

Why? Imagine that you’re shaking a container of mixed nuts. As things jump up, various gaps open up among the nuts. Small items can fall into big or small gaps, whereas big items can only fall into big gaps, which means that small items get more chances to fall; as a result, you end up with the small items steadily moving downwards and the big items steadily rising up on top of them.

This effect isn’t just useful for mixed nuts; it’s useful whenever you need to sort solid items (that can survive shaking) by size. That shows up in everything from manufacturing (either when you want to avoid the effect and keep your mixture together, or when you want to use it to do things like separate gravel) to geology — which is why large chunks of debris end up on top during mudslides.

It also makes for good party tricks, as you can perfectly mix some items yourself while someone else, thinking they’re doing the same thing, always fails. Just make sure you’re loading the big things first, while they’re loading the small ones first.

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