The Use of Physics in Recycling

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Recycling is an important part of looking after our planet. But did you know that we use physics in recycling? Learn about how physics is used to separate materials in recycling plants. Then take a quiz to see what you've learned.

What is Recycling?

In the western world, we use a lot of resources. We have plastic bottles, paper bags, metallic cans, glass jars, and we throw them in the trash at an alarming rate. All of those materials take energy to create, and producing that energy puts greenhouses gases into the atmosphere, damaging our planet. It falls upon us, as humans, to do what we can to protect our world. And one of the ways we can do that is by recycling.

Recycling is the process of turning waste into reusable material. For example, we might melt down plastic to make new plastic items. We can do the same with metal and glass. Paper can be formed into a pulp and turned into lower-grade paper for newspapers or cardboard. Almost everything is reusable if we know how to do it.

Separating Recycled Materials

But before any of that can happen, we have to separate our waste into plastics, metals, glass, and paper. And we do that using the power of the laws of physics: we use gravity, magnetism, conduction, and electromagnetic waves. Let's talk about each of these things, and how they're used in recycling.

Recyclables are sometimes separated by humans to save money and energy
Recyclables are sometimes separated by humans to save money and energy


Gravity is the force that pulls us down towards the earth, and it's probably the simplest way that we separate out recyclable materials. By positioning a series of holes and spinning discs, we can let items filter through something that looks a lot like a giant sieve. Large items make it through larger holes, and smaller items make it through smaller holes. By shaking the recyclables, the heaviest items will fall to the bottom, and the lighter items move towards the top. This separates out metals and glass which are heavier, from paper and cardboard.

Magnetism and Conduction

We also need to separate different types of metal. There's no point melting down aluminum and steel together for example. Why? Well, for one thing, it would produce a result that isn't pure. People don't want steel in their light aluminum cans. But for another, different metals melt at different temperatures, so melting everything together would be a waste of energy.

So we use the laws of physics to help us separate metals too. Different metals have different magnetic properties. Some materials are ferromagnetic, which makes them stick easily to magnets. These items can be pulled out of the mixture by simply moving a magnet nearby.

But other metals are affected by magnets too, just in different ways. All metals conduct electricity, and creating alternating magnetic fields in an area will create electricity inside the metal recyclables. We can use the nature of this electricity to separate the rest of the metals by type using a machine called an eddy current separator.

Sorting machine
Sorting machine

Electromagnetic Waves

Last of all, we use the physics of electromagnetic waves to separate out recyclable materials. This is mostly used for plastics and glass. We have to separate plastics into each different type - for example, polypropylene has to go together, and polyethylene has to go together.

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