Plastic has been the go-to packaging material for many of the snacks we enjoy today, and you’ve likely noticed the unmistakable crackling sound it makes when unwrapped. This noise is particularly noticeable in quiet settings, like a movie theatre or classroom, where the simple act of opening a bag of chips can draw unwanted attention. But what causes plastic wrappers to be so noisy?
What Is Plastic Made Of?
Plastic wrappers, whether for groceries or candy, are made from thin sheets of polymers such as cellophane, mylar, or polyethylene. These materials are popular because they are both lightweight and durable. Despite their thinness, they resist stretching and breaking, making them ideal for packaging. However, their structure also contributes to the loud crackling sound when they are crumpled, which is a phenomenon that lies in their elastic properties.
It All Comes Down To Elastic Potential Energy
To grasp why plastic is so loud, it’s essential to understand the concepts of plastic and elastic deformation. When a material is deformed by force, it behaves in one of two ways: it either returns to its original shape (elastic) or it stays deformed (plastic). Plastic wrappers fall into the latter category, where the deformation becomes permanent.
When you crumple a plastic wrapper, you’re imparting energy into it, which is stored as elastic potential energy. As the wrapper shifts between different configurations, this energy is released, often in the form of sound. If you think back on what you’ve learned about energy in physics tuition, this process is akin to how a bow stores energy when drawn and releases it when shooting an arrow, though in the case of the wrapper, the energy primarily manifests as noise.
The Complex Structure of Crumpled Plastic
A flat plastic sheet is relatively silent because it has a simple, stable configuration. However, when crumpled, it develops numerous creases, each representing a different stable state. Unlike a flat sheet, a crumpled one can settle into various configurations, each holding a certain amount of elastic potential energy. Manipulating a crumpled wrapper shifts it between these states, releasing energy and producing the characteristic crackling sound.
In essence, the noisy nature of plastic wrappers is a result of their physical properties and the energy dynamics involved when they are deformed. Each crinkle and fold contributes to the symphony of sound that makes plastic packaging unmistakably loud.
So What’s Actually Causing The Crunching Sound?
Research shows that the crackling noise of crumpled plastic occurs in distinct bursts or ‘clicks’ rather than as a continuous sound. Let’s delve into what causes this phenomenon.
As mentioned, when a crumpled plastic sheet is manipulated, it transitions between various stable states, each holding a certain amount of elastic potential energy. As you twist or bend the sheet, the energy you exert is stored as potential energy. However, each configuration has a limit to how much energy it can hold. When this limit is surpassed, the sheet abruptly shifts to a new configuration, releasing energy in the form of a ‘click.’
The loudness of each click depends on the size of the crease. Larger creases produce louder sounds, while smaller ones generate softer clicks. As you unwrap a plastic package, it transitions through numerous configurations rapidly, resulting in the sequence of clicks we recognise as the crinkling sound.
A Physics Analogy: The Ridged Hill
Physics experts often compare this process to rolling a rock down a hilly terrain filled with ridges and valleys. Imagine pushing a rock up a hill; when it reaches the summit, it inevitably rolls back down, getting caught in one of the ridges. Each time you push the rock, it may end up on a different ridge.
In this analogy, the ridged hill represents the various stable configurations of the plastic sheet. Pushing the rock uphill mirrors the buildup of elastic energy in the sheet. When the sheet’s energy limit is reached, it snaps into a new configuration, akin to the rock settling into a ridge.
Conclusion
The sound that plastic packaging makes is caused by the sheet buckling from one configuration to another. No matter how slowly or carefully you unwrap it, the clicks will persist. Slowing down the process only reduces the frequency of the clicks, but each step still produces a sound. So, whether you unwrap your snack quickly or slowly, the distinctive crackle of its packaging is something you can’t avoid.
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