# Elasticity

Some people worry about wrinkles but therein lies the irony: the more you worry and frown, the more wrinkles you are likely have! We don’t necessarily think of it that way but skin is an elastic material which, gradually, turns into a plastic material. That’s right, plastic.  As the skin ages, it becomes less elastic and more plastic. What does that mean, you ask?

Solids can be categorised according to how stretchy they are. If you take an elastic wrist band and compare that with paper, for instance, then you could stretch the elastic band to a certain extent without it snapping but if you try and pull on the piece of paper, it would rip. You can already observe the difference between those two kinds of material. One has more give and can easily be pulled and stretched while the other snaps or rips just as easily.

Skin and wrist band elasticity

We can draw this line and mark one end as zero and the other as one and call it the ‘elastic scale’. Things which are very elastic will be put nearer the end marked 1 while things which we aren’t very elastic will be nearer the end marked 0. Things which are not elastic are called plastic. That is to say, things which cannot easily be deformed or bent or stretched are deemed ‘plastic’ materials. Glass, for example, is one such material. Try bending a sheet of glass and it will snap. Of course, be careful with that as you might hurt yourself but you must have seen or experienced how plastic glass is, how brittle it is.

What we typically call ‘plastics’ (bags, bowls, bottles et cetera) are not usually very stretchy materials. Some plastic bags can be stretched before they rip but that doesn’t mean they are elastic. You see, what we mean by ‘elastic’ is not just how much it can stretch but also whether the material goes back to how it was before it was stretched. Given a piece of elastic band of a given shape and length, you can stretch it and let go and it will return to its original length and shape as though it didn’t suffer any deformation. However, it you try and stretch a plastic bag, its handles might elongate but, once you let go, they won’t return to their original length and shape. This is the difference between ‘elastic’ and ‘plastic’ materials.

Some plastics can be elastic as well. What I mean here is that some materials which we usually call ‘plastics’ (such as sheets of Perspex or bottles) can be a little bit elastic in nature. That is, they can be deformed (to a certain extent) either by squeezing or bending and they will return to their original shape. You can experiment on this elastic nature of ‘plastic’ bottles by squeezing them and letting go to see if they return to their original shape.

Fair enough, calling an object ‘plastic’ when it can display elastic characteristics can be quite confusing. But this is one example of where the crossover from one lingo to another (from physics to english) comes with some ambiguity. Another example is the word ‘weight’. What we mean by ‘weight’ in physics is different from its common, every day meaning. In physics, ‘weight’ is a force whereas in our daily parlance it has to do with the amount of stuff or mass. What we mean by ‘plastic’ in physics is not necessarily those every day objects we usually refer to as plastics. Plastic, in physics, is about the property of a material as opposed to what type of material it is. That is, if we consider a metal ruler, a ceramic vase and a wooden chair then they are all made up of different types of materials: metal, ceramic and wood. But they also share a common property. They all have a certain amount of ‘plasticity’. Now, if we consider the opposite of this ‘plastic’ characteristic, then we can associate a certain amount of ‘elasticity’ to these different objects. Typically, if something is nearer the 1 in our scale, we’ll refer to it as ‘elastic’ rather than to ‘not-very-plastic’. We could have devised a ‘stretchiness’ scale and define 1 as being completely not stretchy or plastic and 0 as completely elastic. This would not change the fact that some stuff are more plastic than others and that not everything we call ‘plastic’ in everyday life are truly plastic. For now, we’ll stick to the scale where 1 means completely elastic such that no matter how much you stretch it, it comes back to its original state. It simply cannot be deformed. no matter what. And on our scale, 0 means it is completely plastic such that it remains deformed even if the deformation is incredibly small.

To come back to our skin, its elasticity changes with our age. The younger we are, the more elastic it is; our skin would be closer to the 1 on our scale. As we age, it moves towards the 0 mark. If we pinch the skin on top of our hand and pull, it will restore its position and shape when we let go, almost immediately. That means we have a young, elastic skin. If it takes a long time to return to its original shape (or in some cases it might not completely revert to its original shape) then our skin is old and less elastic; it is plastic.

The elasticity of a material is a physical quantity that can be observed, measured and experimented with. As such, we can devise a scale, just like we did above and classify different materials according to their degree of elasticity.

We should be careful, here, not to confuse the strength of a material with its elasticity. It is easy to snap a biscuit in pieces but it would take much more effort to snap a poker chip of the same size and thickness. Both are plastic objects in that they do not return to their original shape when deformed but that does not mean they are equally strong materials.

Likewise, a sheet of rubber might be easily stretched and pulled in all direction but it can be ‘stronger’ than a sheet of glass in the sense that it would be easy to break the glass. Being less deformable doesn’t necessarily equate to being stronger. Some materials can be tough yet brittle while others can be malleable yet plastic. If you consider a small piece of aluminium sheet, you can easily bend it, twist it, reshape it; it is very malleable. But it is not an elastic material because after its been deformed and bent and twisted, it doesn’t return to its initial shape. As you can imagine, there are all sorts of materials with varying properties such as elasticity, strength and malleability.

What makes them so different? What makes glass so brittle yet rubber is so elastic? It has to do with how the molecules of these materials are configured. As we’ve already seen (please refer to Matter), stuff is made up of small building blocks called atoms. Amalgamation of these atoms make up what we call molecules. Now, depending on how the molecules are bound together, the different materials will have different properties. In an elastic band, for example, the molecules are like long chains coiled up on each other, multiple times. As we stretch the elastic bands, those coiled up molecules unfurl, uncoil and line up as opposed to being lumped together in this big bundle of coils. But when we let go, those unfurled molecules come back again to their coiled up state because they are still linked to one another, still pulling on each other with intermolecular forces. This is what gives elastic bands their elasticity, their ability to stretch. Think of a spring. Those molecules are in a similar shape. You can pull and push on a spring and it will lengthen and shorten but always come back to its initial length as long as you don’t pull or push it too hard that it breaks.

The molecules in a plastic material are not coiled up in that way. They tend to be shorter, less linked to one another, more in layers than in bundles. So when these molecules are displaced from their initial configuration, they do not tend to go back to where they were because there are no forces pulling them back to that original state. Instead, they adopt a new state, a new configuration and stay there. These molecular structures are more crystalline in nature. That is to say, they are stacked in blocks rather than coils, to put it simply. Crystals, like quartz for example, are such types of materials. They might be tough but they’re plastic and brittle.

As mentioned before, the elastic property of materials can be analysed and understood quantitatively. We can determine the amount by which a certain material will be deformed if we apply a given force on it. We can calculate how much force we need to break a certain material. We can determine the maximum load a given spring can sustain before it becomes plastic. All of these can be figured out if we know the elastic properties of matter.

There are essentially two ways by which the elasticity of an object can determine how it will behave under an applied force. Picture a metal rod of a given initial length and diameter. One can apply a force at one end of the rod and either compress it or extend it by a certain amount. The amount by which it extends or compresses compared to its initial length is called the strain on the rod. The force applied over the area of the circular end of the rod is what we call the stresson the rod. Strain and stress are related to each other according to the elastic properties of the material in question. If we represent stress by σ and strain by x then what we have is the following simple equation: σ = Ex. Here, E represents Young’s modulus of the given material. Young’s modulus is just another term for the stiffness of an elastic material and is named after Thomas Young, a scientist and polymath who, amongst other things, studied the elastic properties of matter. He was able to determine that the stiffness of the material, and hence its given E value, was not dependent on the shape or geometry of the material. What mattered was the type of material it was. So, whether we have rod of steel or a block of steel then both of them will have the same stiffness, the same E. That was a leap forward in mechanical engineer in understanding how materials behave under different forces and it made life simpler in that we had one less thing to worry about (the shape of the material) for what mattered was the material itself.

Stress, as you can see, is a physical quantity which, I think, has been appropriated by the psychiatrists to define how we react under a lot of pressure, mental pressure. You see, stress which a material is subject to is physically equivalent to the pressure acting on it. Pressure, if you recall (please see my blog on Pressure) is defined as the force acting on a given surface area. Similarly, the force acting on a given cross-sectional area of a material is known as the stress on the material. Mental stress, it seems, is not unlike this physical counterpart. If one person had to handle hundreds of different requests at the same time then it is likely that the person will feel a lot of pressure and therefore be stressed. However, if those requests are divided among a group of people such that each person is only handling a few requests then the amount of pressure or stress on that person will be less. I agree that this is a very rudimentary analogy but it helps understand the similarity of the two situations. Mental pressure is not so different from physical pressure in that sense. By spreading the ‘surface area’ over which the force is acting, by distributing the requests among many more people, the pressure or stress is relieved. The mind, I believe, also has its ‘elasticity’. In the sense that, if the stress is too high then one could snap and not be able to cope with the demanding situation. Just like an elastic band could snap if pulled too much, the mind is no different in this manner. Now, I’m not suggesting that these two systems behave the same or that there is a mechanical explanation to mental stress. Nevertheless, there is an analogy, albeit basic, between what we, physicists, call stress and what psychiatrists, call stress.

To come full circle on this, therefore, if one were to worry less, or be less stressed, then one could reduce the chances of wrinkles appearing even if our skin will inevitably become less elastic as we age. The skin’s elasticity and the mind’s ‘elasticity’ could perhaps be related. I’ll leave that as an open question…