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Defining quantities: weighty and stressful issues

The International Space Station, cause of many an argument about freefall and weightlessness. Credit: MarcelClemens/Shutterstock/NASA

Earlier this month we had another one of our glossary meetings, in which we try to define quantities for use in schools and college-level physics.

The project was initiated by a discussion on one of our online teacher discussion forums about how emf should be defined, and how this was inconsistent between different exam boards. Further discussion led to the discovery of a number of other anomalies. We felt it was time to address this.

When I signed up, I thought that, once we had put together a working group of educational experts, we would knock out some definitions in a few months. That was more than two years ago.

A major hurdle is that some quantities just can’t be defined properly without university-level maths. How can you fully define stress without talking about tensors? Another problems is that we use the same word to mean different things. A case in point is weight. I always thought of weight as a gravitational force, but others define weight as an upward contact force. If it’s a gravitational force, how high-up can you go before you stop calling it weight? We don’t call the gravitational force on the Moon due to the Earth weight, but what about that on the International Space Station? Also what do we mean by weightlessness? Is it a region where the gravitational force is zero or is a person in free-fall also weightless? Different people have different takes. And don’t even get me started on energy.

It’s been a long, hard slog: sometimes enjoyable, sometimes painful – always taxing. We now have almost 44 quantities defined, along with units and explanations of how these are used in context. Although we still have quite a few more to do, it is getting there. We will soon be releasing some of our definitions into the wild. No doubt we will encounter disagreements, and there will be number of iterations. It will be a marathon rather than a sprint, but the goal is worth it. If we, as an educational community, can agree a set of definitions, students will no longer have to rely on the first website they happen to come across. They will have a definitive online reference guide that they can have confidence in – one that we hope will be adopted across the educational community.

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Taj Bhutta

Taj Bhutta

IOP school support manager
Taj Bhutta

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3 thoughts on “Defining quantities: weighty and stressful issues

  1. Hi Taj

    Interesting post – thanks.

    For teachers, the words; accuracy (not too bad), precision and reliability seem to be defined by examination boards and particularly “precision” redefined at will. For learners requiring to commit a body of knowledge to memory – definition matter, especially when they are somewhat fluid.

    Glen Gilchrist

  2. Thanks Glen. Although we are concentrating on quantities at the moment – purely for our own sanity – you make a good point. Perhaps to be added to list of definitions at a later point.

  3. Accuracy, precision and reliability were defined in the Nuffield and ASE publication, The Language of Measurement, which can be purchased from the ASE bookshop.

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