This year I taught 45 Year 10 pupils Newton’s Laws of Motion. After lots of assessment in the classroom, the majority of the pupils appeared to be able to correctly identify Newton’s Third Law pairs of forces. I warned the classes I taught this year, “you think you get it, but most of you don’t.” It sounded harsh and unfair. They were able to identify the pair of forces in all sorts of situations, job done, mission accomplished, so why would I arrogantly proclaim that they “don’t get it?” In the End of Year examinations I included a question where pupils had to describe the Newton’s Third Law paired force to the weight of a person jumping on a trampoline. Out of 45 pupils, only one got the answer right.
Perhaps that means I’m a terrible teacher? Well, out of nearly 200 pupils in Year 10, only four got the answer right. So it’s not me.
What is going on here? Why do pupils find it so difficult to understand Newton’s Third Law? The problem, as I have often said, is that pupils are taught the worst possible wording for Newton’s Third Law from a very early age, and it sticks. The worst possible wording of Newton’s Third Law is, “every action has an equal and opposite reaction.”
That wording creates misconceptions. The phrase is applied to all sorts of situations with no relevence to forces at all. Children hear the phrase spoken by parents, siblings, friends, relatives, people on the TV, all over the place. They read the phrase written in poor quality revision guides and textbooks, notes, workbooks, the list goes on and on. They think they understand what Newton’s Third Law is because they can recite that phrase, but that phrase does not adequately describe Newton’s Third Law at all.
Action in Physics refers to the integral of the difference between the kinetic energy and potential energy for mechanical system with respect to time. That’s probably not what people mean when they say “every action has an equal and opposite reaction”.
In this video, embedded above, I’ll explore what Newton’s Third Law really means, and give some pointers to help people spot Newton’s Third Law pairs. You’ll watch the video, and you’ll think you get it. I hope you do get it.
Experience tells me you won’t, but I won’t give up trying to help you with it. One day the penny will drop, and forces will all click into place.
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