Over the past century it seems as though all of the big discoveries in Physics have come from either Relativity Theory or Quantum Mechanics. These two pillars of 20th century science are all about objects that are moving very, very fast, or are very, very small. Sometimes it seems as if the old Physics of Isaac Newton has little left to teach us, as if we’ve learned everything there is to know about the behaviour of objects in our everyday world. In this post however I’ll be discussing two recent studies that show how much we still have to learn from classical physics about the ways objects in our everyday life behave.
The first paper I’ll be discussing comes from researchers at the University of Rennes and the University of Lyon, both in France along with Aoyama Gakuin University in Japan who examined the different shapes and forms that knitted fabrics can take on after being stretched and pulled. Specifically, the team used a common jersey knit stitch known as stockinette, which consists of interlocking loops of threads, to knit a piece of fabric with 70×70 stitches. See figure below. This piece of fabric was then placed on a specifically designed tensile mechanism that allowed the researchers to stretch, pull and twist the piece of fabric in a wide variety of different directions and strengths.
Now we all know that knits that are really pulled and stretched never quite return to their original shape, they become deformed. What the researchers did however was to measure the degree of deformation that resulted in their piece of fabric depending on the amount of stretching given to the fabric. Each of the resulting shapes that the piece of fabric took on after stretching and twisting the team designated as a ‘metastable shape’ and they categorized the many metastable shapes generated during their testing.
At the same time the team ran a series of computer simulations that replicated the actual experimental results. One thing the computer simulations allowed the researchers to do that they couldn’t do experimentally was to reduce and even completely eliminate the effect of friction between the stitches of their piece of fabric. What the team discovered in these simulations was that, in the absence of friction the piece of fabric always returned to its original shape, regardless of the amount of stretching and twisting. Without friction there were no matastable shapes.
Perhaps the research conducted by the team in France and Japan will help manufacturers develop clothing that does not lose it shape after being worn or washed, perhaps not. At least however you now know that friction is to blame when your favourite sweater gets deformed.
Another study dealt with a twist on the familiar phenomenon of how two or more objects moving a fluid, water or air, actually reduce the force of friction on each other. We’ve all seen how a flock of geese will fly in a ‘V’ shaped pattern. Well that’s because the lead goose’s motion sets up an flow of air called a bow wave that reduces friction to the two geese on either side of it and slightly behind, reducing the amount of energy they have to expend in flying. This reduction in friction continues right down the line so that the flock forms a ‘V’ shape in order to reduce the amount of energy they expend in flying. In water dolphins will often swim close the bow of a ship to take advantage of the same phenomenon, and many species of schooling fish arrange themselves for the same reason.
Obviously this doesn’t work in a solid medium because solid objects simply cannot move through a solid medium. What about a granular medium however, where each individual grain may be solid but where thousands, if not millions of tiny grains can still in many ways behave like a fluid.
That’s what physicists at the University of Campinas in Brazil and the University of Paris-Saclay in France decided to study. The experimental setup the researchers employed consisted of a bed of glass beads, used in place of sand because of their uniformity, through which two steel balls called ‘intruders’ could be pulled in parallel. The researchers could vary both the distance between the intruders as well as their depth in the glass beads, riding the surface, just submerged or fully submerged etc.
What the team discovered was that there was a significant reduction, nearly 30%, in the force of fiction on both balls when they were so close as to be almost touching. The cause of this reduction in friction the researchers attribute to the motion of one intruder breaking the force chains between the grains around the other intruder, and vice versa.
The researchers also believe that their findings may help to explain some well-known phenomenon in the natural world such as the digging of animal burrows and the growth of plant roots. In any case the results discovered by both teams of physicists clearly show that classical physics can still teach us a lot about the world around us.