A favourite pastime when I was little was playing with salt and sand.
This usually entailed pouring out the whole house supply of, in particular, domestic white table salt onto a dinner tray and spending hours making trails through it with a toy motor car. Sand was similarly spread about on a worktop indoors or on top of a paving slab outdoors and coursed through with more vehicles or a handful of plastic soldiers. Sand was readily available in our street where there was always new house construction under way. I justified taking a few loads in my plastic seaside bucket in that if left in situ it would only develop into a large litter tray for the neighbourhood cats.
These two main granular materials were not the most satisfying that could be used because they were subject to the science, physics or just natural phenomena of the angle of repose. If I had been familiar with this principle at under ten years of age I may have not even bothered to attempt to use them as play media and perhaps opted for something with a bit more mass, pliability and scope for fun.
Apparently the study of the characteristics of granular materials is quite a wide discipline with significance to an extent of which I had no knowledge. It features in many day to day things from civil engineering projects through to manufacturing, bulk storage and production, geology and safety and even in the natural world.
The scientific explanation is simply that when materials such as my two favourite childhood playthings but also foodstuffs from wheat to bran, shredded coconut to wheat and malt to coffee beans are poured out onto a horizontal surface a conical pile will form. The angle of repose will determine the stability of the material or how much friction there is between the surfaces and shapes of the particles. In mechanics the same applies and refers to the maximum angle at which an object can rest on an inclined plane before it begins to slide.
Of course every small child knows this to be fact, even if not fully cogniscent with the theory. For example, my childhood trike, if left on the ramp of the driveway was always destined to break free of any friction with the surface and roll out into the public highway causing an obstruction and downright hazard.
Having in my later years read a bit more about such scientific matters I have been amazed by the adoption of the angle of repose in nature.
Some of the smallest and most sedate of creatures have, by harnessing the theory, ensured their survival and indeed have flourished and multiplied in numbers and health in their environments which have been under constant threat from predators and Man.
The incongruously but under close scrutiny aptly named antlions and wormlions excavate conical shaped pits in the loose sand of their habitats. Their efforts involve flinging loose sand up the incline of the pit so that it settles and initially stabilises at the critical angle of repose. The materials are therefore sensitive and stable until the smallest of insects unwittingly enters and finds itself activating the inherent instability which causes the victim, shortly to comprise lunch, to cascade into the base of the pit where the expectant diner awaits. Even if the visitor seems to have resisted tripping the trap the host can project loose sand grains up into the mouth of the inverted cone which acts as a catalyst to encourage the tit-bit to tumble to its fate.
Now, I wonder what the angle of repose was when I fell off that easy chair in the garden . For a chubby male aged 52 years it looks to be about 180 degrees.
(previously published on this day some years ago now)
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