How Elastic Bands Work

I was roaming across YouTube when I came across this VSauce Lean back (a playlist of videos chosen by VSauce). Now the first video of the Leanback was staring Richard P Feynman as he was explaining how a simple thing as rubber band works. So, in this post I am posting on how rubber bands work in my own words.

First of let us understand, what is elasticity. Elasticity is the property of a material regaining its physical shape and size after being stretched and/or squeezed. It is shown by almost all metals and some other materials as well. But NOT rubber bands.

In fact, rubber bands or elastic bands are not elastic at all. They are plastic. Plastic substances are those that do not regain there original shape after being stretched and/or squeezed. Wait, what?! Doesn’t rubber band do that? Well, no. Even though it seemingly regains its shape, the shape it regains is not the original one. It has undergone small amount of permanent set. Permanent set means a substance has lost the ability to regain the original shape. If a true elastic material undergoes permanent set, it won’t be regaining its original shape completely and that object becomes completely or partially plastic. 

There is one more reason for calling elastic bands not elastic. The reason for an elastic band regaining its original shape is not the same as a true elastic material. A true elastic material regains its shape purely due to pull between the molecules.

However, elastic bands work differently. Elastic Bands tend to heat up when they are stretched. The heating up causes the molecules to vibrate even more and hence we experience a pull. It is similar to a vibrating string. Try pulling a strong taut and move or rather hit the middle portion of the string and we will experience a pull. Similarly, the motion of the molecules in the band and the band hitting the nearby molecules causes the rubber band to exhibit the pull that we feel. Here, the motion of the vibrating (or as Lord Feynman puts it jiggling) molecules the pull is experienced. Hence, the stronger we heat the band, the more the vibration and consequently, the stronger the pull. 

When we release the band, the heat is used up in pulling the molecules together. The molecules cool down and slowly rearrange themselves as they were except for small changes. Here, there is an action of the force between the molecules. This force is proportional to heat. This increase in force due to hearings exactly the opposite of what happens in the case of true elastic materials. In true elastic materials, heating will actually case the material NOT to regain its shape. 

The rubber band actually heats more upon being stretched, vibrates more and hence, pulls more. True elastic materials have the same property of “as the extension, so the pull” (Robert Hooke on Elasticity) but it is not related to heating, or vibrating molecules. They just have an intermolecular pull, which is pretty boring.

We can actually feel the heat change by touching the rubber band while stretching it and releasing it. This property is actually a pretty good example of how a pretty simple thing that we use everyday can be different from our normal perception and can spark interest.

Feel free to leave your comments and tell me there is more to the working of an elastic band and if there is some other thing of daily use that is actually different from our own perception.



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