# Newton’s third law

By Marcus Wilson 27/07/2010

In yesterday’s tutorial I had an enlightening discussion with the students about Newton’s third law. Enlightening for me just as much as I hope it was for them. You’ll find the law in textbooks phrased something like "for every action there is an equal and opposite reaction".

Sounds simple, and to someone who is well trained in physics, it is. But to others, as I found out, it is intensely confusing. We did some examples (prepared by a well-known science publishing company) designed to elucidate the law. This draws out things like:

1. The action and reaction are on different bodies

2. The size of the force on body 1 due to body 2 is equal to the size of that on body 2 due to body 1 (An ‘equal reaction’)

3. The directions of the two forces (remember, forces are vectors) are exactly opposite.

4. The action and reaction are the same ‘kind’ of force. Eg if body 1 exerts a frictional force on body 2, body 2 exerts a frictional force on body 1.

Having gone through all these, the students all appeared to be understanding it. Except they didn’t. I know they didn’t because I then gave them a practical example – explain how one can walk on a floor but not very easily on ice – and suddenly points 1-4 all went out of the window. Instant forgetting. I was hearing things like – you put a force backwards on the ice with your foot but the reaction is now perpendicular and your foot slides sideways.  Perpendicular? What happened to ‘equal and opposite‘?   Or ‘you put a force on the ice but because its slippery it can’t put a reaction back on you?’. Hmmm. What about the forces being ‘equal and opposite’?

What does this tell me? Well, first, that Newton’s third law isn’t the easy-peasy physics that I might think it is. That one simple sentence hides a series of quite tricky concepts. Secondly, teaching it in one tutorial is wishful thinking. (Actually, this was just one question out of four that I was supposed to cover here – WAY too much for a session). And third, student discussion is really, really valuable. Both to them, and to me, As a group, they were beginning to home in on the correct understanding after much discussion (good for them),  and their comments were telling me that they still didn’t get it (good for me, because it now guides my teaching, which, of course, ends up being good for them).

I give the same tutorial to a different group on Friday, and I think I’ll abandon the other three questions and focus on this one.

Students – apologies if I’ve misrepresented your comments. I’m typing from memory. I did find our tutorial very, very enlightening, so thank you for teaching me how to teach. (Even if you didn’t realize that was what you were doing).

### 0 Responses to “Newton’s third law”

• possum says:

It seems that the mention of ice led instantly to thoughts of “slippery”, whereas you meant “weak and easily breakable”. Maybe changing ice to window glass in your example might help the students focus on your point.

• Marcus Wilson says:

Well, actually I did mean slippery, but yes, people can easily jump into thinking about certain aspects of the problem.

Why can’t we walk on ice (assuming its strong enough to support us)? There is very little friction between our foot and the ice. We try to push backwards on the ice with our feet (as we would walking along the ground) but we can exert very little force back on the ice, so, the ice’s reaction on us, being equal and opposite, is very little too.

• possum says:

Ah, now I am with you. You hadn’t mentioned coefficients of friction and I easily jumped into assuming it was the vertical forces you were thinking of.

• ross says:

An even more facinating thought experiment is to ask them as a conequence of the equal and opposite reaction:

You are in free space in a rocket and you fire the thing off. You disappear into far far space in a nice “straight line”, where does the centre of gravity of the system end up?