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Brazi Nuts rising to the top

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849590.  Fri Sep 23, 2011 6:41 pm Reply with quote

Not sure if this is the right forum to add this, but here goes anyway:

Surely the reason that Brazil Nuts rise to the top in a bag with other nuts is that they are bigger than all the other nuts, in much the same way that all larger things rise to the top when mixed with smaller things and shaken. I've done this experiment many times (both virtually in LittleBigPlanet and in real life) and the results almost never vary.

I'm pretty sure it's slightly hyperbolic to say that nobody knows something as elementary as this.

849593.  Fri Sep 23, 2011 6:46 pm Reply with quote

It's not as simple as that Sgt - if you take a tray of sand and boulders and shake it, the boulders will inevitably worm their way to the bottom.

849596.  Fri Sep 23, 2011 6:56 pm Reply with quote

Perhaps the weight of -boulders- compared to -sand- counteracts the effect. I've never tried with anything bigger or heavier than somewhat large rocks in sand, but the point still stands, since brazil nuts are nowhere near as heavy as a somewhat large rock, and the other nuts in the bag are much heavier than grains of sand. :)

The only times when I haven't achieved this effect is when I haven't bothered to shake for long enough. (yes, I tried many times with many variables, because when I heard of this from my physics professor, I didn't believe him.)

849597.  Fri Sep 23, 2011 7:07 pm Reply with quote

That's the whole point Sgt. In some circumstances smaller particles fall to the bottom, in other circumstances larger particles fall to the bottom. Nobody knows what the dynamics are which determine which takes precedence.

849599.  Fri Sep 23, 2011 7:23 pm Reply with quote

One source for this was Ulrich, S et al (2007). 'Influence of friction on granular segregation', Physical Review E 76(4).

Dr Ulrich and his team at G÷ttingen used a mechanical shaking device to agitate a mixture of glass beads and (larger, heavier) brass beads. Initially, the brass beads sank to the bottom, but there was a sharp transition after 25 hours of shaking - many of the brass beads came to the top.

Interestingly, extending the shaking period further yet - in one case, he left the machine running while absent for a week - didn't bring even more to the top.

Washing the beads and their container with soap and water did not cause the brass beads to return to the bottom, but washing them with a heavy duty industrial detergent (Alconox) did.

849733.  Sat Sep 24, 2011 8:45 am Reply with quote

Walk any beach exposed to wave action from the sea, and you will see this sorting process laid out: larger boulders at the top of the beach, grading down to sand in the littoral (between tides) region. This is very noticeable on the less frequented beaches of the West coast of Ireland; except where very large rocks and concrete blocks have been placed to reduce erosion.
The action of the waves is continually lifting solids from the bottom and rotating them; as the rollers* break in shallower water, the suspended solids are deposited.

*roller is the good old name for unbroken waves, and very apt it is, for we now know that particles in a water wave only rotate, and do not move forward.

849781.  Sat Sep 24, 2011 12:09 pm Reply with quote

mckeonj wrote:
*roller is the good old name for unbroken waves, and very apt it is, for we now know that particles in a water wave only rotate, and do not move forward.

Only when modelling linear waves in deep water - for a start, in shallow water the particles move in elliptical paths due to the effect of the sea bed.
And even if you were referring to deep water waves, the assumption that water particles move in a circular path only works when assuming a linear wave model - in reality, the water particles can move in the direction of the wave, or in the opposite direction, when Stokes drift is taken into account.

Phil Rogers
849826.  Sat Sep 24, 2011 4:09 pm Reply with quote

During the Brazil nut discussion, Stephen mentioned that it is used in the purple Quality Street.

This is no longer correct. Although the chocolate is still shaped like a Brazil, it now contains hazelnuts.

849828.  Sat Sep 24, 2011 4:14 pm Reply with quote

I thought the purple ones had always been hazel-in-caramel and that none of the chocs had brazil nuts in them. Maybe I'm getting confused with Roses...

849831.  Sat Sep 24, 2011 4:22 pm Reply with quote

It's a little more complex than the size of the particles, or the weight/density of the particles.

It's down to the density of the way they pack and fit together.

849838.  Sat Sep 24, 2011 5:04 pm Reply with quote

Not only are Brazil nuts not nuts, but Brazil is not the largest producer of them. Just over one half of Brazil nuts are grown in Bolivia, where they are known as casta˝as de Beni = Beni chestnuts. (Beni is a region of Bolivia)

They are also known in Bolivia as almendras - although this is somewhat confusing, because to most other Spanish speakers that word refers to almonds.

869232.  Wed Dec 07, 2011 11:57 pm Reply with quote

This has been explained by physics for sometime now.

Just watch this link on Youtube it will explain it:

I hope they bring this up on the show as I would hope QI's researchers do a bit more research then a Google search...

869275.  Thu Dec 08, 2011 5:34 am Reply with quote

Welcome to another inhabitant of Planet Oz

881325.  Sat Jan 28, 2012 2:06 pm Reply with quote

I will watch the video shortly but I always understood it like this.

If you have items of the same size/shape and different densities then the heavier ones will sink and the lighter ones rise.

In a mixture were the weights are the same but the sizes different then it becomes a more complex process. If a large item shifts, a smaller item can get under it. However it requires several small items to move at once to allow a larger one to sink. The result is that the larger ones rise.


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