Sir Isaac Newton

Definitio. I.

Quantitas materiæ est mensura ejusdem orta ex illius densitate et magnitudine conjunctim.

Definition I

The quantity of matter is the measure of the same, arising from its density and bulk conjunctly.

                        (Newton, translation by Motte)

This is Newton’s definition of mass. Newton had to be extremely clear about his terms: many of them were new. He needed definitions to communicate clearly with professional scientists. This is what definitions are for.

There is a tradition of using definitions in science education. In modern times, the core knowledge movement has embraced definitions – you can prove that a learner has learnt them.

Illustration by Mary Lee

But using definitions for learning has problems. A good definition has a subtlety and precision of language that makes them inaccessible to learners. You need to understand the concept before the definition is useful.

The definition of mass that I use is: mass is the resistance of a body to acceleration. But by the time you have understood the term acceleration, you’ve got a good grasp of mass already.

So I don’t give students a written definition of mass and expect them to be able to do anything with it. Instead, when I teach mass, I get students to experience it.

I need to keep mass and weight separate in my students’ minds, so everything has to happen horizontally. I get them to push a trolley with masses on – I ask them to accelerate and decelerate the trolley with their hands. Everyone has to do it. They get to feel mass as a resistance to changing speed. It’s a different sort of knowledge – it is still core knowledge, but it isn’t a definition.

Using written definitions takes some thought.

Definitions are useful when the meaning builds upon concepts learners already have a strong grasp of. Momentum, for example, needs a definition: it is the product of mass and velocity. You don’t teach momentum before students know mass and velocity (there is a well established sequence of teaching science, developed over centuries). For teaching, definitions need meaning.

“I am implying – well, no, I am stating – that every teacher should get his or her students to think about the meaning of material.”

(Willingham – Why Don’t Students Like School?)

Every lesson I saw this week and last had a section on definitions. The method I have seen most has been:

Define the concept in your own words.

Look at an ‘official’ definition.

Part 1 is an attempt to get students to think about the meaning, but it is very challenging. I think too challenging. The writer needs to have a very clear overview not only of the phenomena, but also the limits, context and potential misunderstandings of the phenomena. Before you can define a word, you need an expert’s understanding of it. It is far more likely that this exercise will have students practising (and learning) something incorrect.

Part 2 involves reading, but typically not deep reading, so is a far less memorable activity. And memory is what we are aiming for.

My first rule of definitions is to ensure that learners already have a decent understanding of the concept.

My second rule of definitions is use a good one. Don’t ‘invent’ one on the fly: it takes time to refine a decent definition and you want a consistent definition between teachers and across years. It makes sense to use the exam board definition if available.

Here is a different model for teaching definitions:

  • Experience the phenomena – erosion, rotation, current. This could be through practical work, simulations and models, video etc.
  • Discuss the phenomena. Assess whether your learners have seen what you wanted them to see (plan this section – ask really good questions and make sure everyone can answer).
  • Show the definition you want them to learn.
  • Read the definition closely. Explore the clever wording. What does it say? What doesn’t it say? Is it perfect? What words did the writer use? I use sentence starters to prompt questioning, evalaution and rewording: I wonder….. In other words…. This would be better if…


Definition: Gravitational potential energy – a name used to describe energy when it is stored in objects in high places that can fall down.

(Exploring Science 7)

Student 1: I wonder what happens if it can’t fall down?

Student 2: In other words, objects that are high up and can fall down have gravitational potential energy.

Student 3: This would be better if it explained how the energy was stored in the object.

  • Practise recalling the definition: call and response; catch one partner; memory games…
  • Read the word in texts.
  • Practise using the word in sentences, verbally and in writing.

(Current) To make the bulb brighter, …..

(Rotation)  You need a fixed point for rotation, because….

  • Encourage the use of the keyword in longer explanations.
  • Practise using the verb/noun/adjective form of the verb (e.g. reflect, reflection, reflected).

Learning takes repetition: if want your students to commit the definitions to long term memory, you will have to revisit these activities several times. You should check the retention over weeks and terms.

In summary: definitions are worth learning if they mean something to the learner.

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