While I’m on vacation, I’m reposting older posts from my archives, in the hopes of generating a bit more discussion, and helping me to semisconceptionse how much I’ve grown. Now that I’ve been provoked to think differently about by Brian Frank, I find this post especially amusing.

One of the other great lessons I learned from Arons is that lanugage matters. Take the classic formulation of Newton’s first law:

An object at rest tends to stay at rest, while an object in motion tends to stay in motion, unless acted upon by a net force.

Right here, this statement is setting kids up to misunderstand the idea of inertia, just by setting up some sort of dichotomy between being at “rest” and “in motion.” The big idea really is both of these states of motion are constant velocity and

An object’s velocity will remain constant unless the object is acted upon by a net force.

See how much clearer this is?

The same thing can be said with the usual formulation of Newton’s 3rd law:

For every action, there is an equal and opposite reaction.

Hey, I’m just starting to figure out what a force is, and you mean to tell me there are actions and reactions too? Which one is which? What’s the difference between an action a reaction and a force? This brings up the idea that when you use new words (action and reaction) to describe things that you can describe equally well with old words (force) you really only add to confusion.

So let’s reformulate N3 like this:

If object A exerts a force on object B, then object B will exert a force on object A. These forces will be of the same type, equal in size and opposite in direction.

There, much clearer, and if you have mastered how to name forces (Not gravity, but the gravitational force of the earth on the ball), mastering N3 is as simple as switching around A and B.

This also brings me to another point—in our modeling discussions, kids love to bring up all sorts of complicated explanations for the simplest of phenomena (“The ball has run out of momentum, and that’s why it stops”), or they use my most hated word, deacceleration (does this mean negative acceleration? Or slowing down? The two aren’t the same, and students easily think they are).

I used to simply ban words like deacceleration, and ban the use of ideas we hadn’t studied yet (I would even administer an oath–“I, Milhouse, promise never to use the word deacceleration again”). Now, however, I tell them a story. Imagine your 7 year-old cousin, learning softball for the first time, and you want them to bunt. But bunt is a complex idea, and they aren’t really going to know what you mean by saying “hey, Cleo,  bunt the ball!” Instead, it would be much clearer to say “hold your bat still right in front of the ball.”  This has helped the students to see why all the fancy words they might know may be very useful in the future, but now, when we’re just learning the rules of the game, can tend to make things overly complicated and opaque.

ps. Don’t think N2 escapes my need for revision as well. Thanks to PSSC, I think it makes far more sense to say

$a=\frac{F_{net}}{m}$, which separates the changes in motion, acceleration, from the things that affect motion (inertia and force).