For every action there is an equal and opposite reaction.

— Newton’s Third Law of Motion

I introduced my Middle Schoolers to the principles of Newton’s Laws of Motion last week.

The discussion started off with projectiles. If you’re floating in space — zero gravity — and throw something, like a basketball, away from you, you’ll be pushed off in the opposite direction. In fact, if you throw something that has the exact same mass as you do away from yourself, you’ll move off in the opposite direction with the exact same speed as the thing you threw.

Then I brought up rockets, and how they’re expelling gas to move them forward. I think it was the phrase, “expelling gas” that did it. The next question, which the student brought up somewhat circumspectly, sidling around the issue and the language, was (more or less), “So if you expel gas in space will you move off in the other direction?”

The simple answer, appropriate to that stage of the discussion, was, of course, “Yes.”

Which lead to to, “What about spitting?”

“Yes.”

“What about, you know, peeing?”

“Yes, except …”

At that point I thought it would be wise to rein it in a little, and make a further point about the whole action-reaction thing.

“You see, if you expel anything, wouldn’t it just be stuck in your spacesuit with you? Then you’re not really expelling it, it’s still attached to you, so you wouldn’t really move. What would be more useful would be to collect it in something like a spray can or a squeeze bottle. Then you can just squirt it out opposite the direction you want to go in to control your movement.”

This produced a moment of thoughtful silence as they figured out the logistics.

Notes

I thought this was a useful conversation to have. The students were interested and animated. And I believe it’ll be memorable too.

P.S.: I’d wanted to talk about ion drives, which operate on the same reaction principle, but are much cooler (after all they’re used in Star Trek). Instead of burning fuel to create the propulsive force ion drives generate an electric field that ejects charged particles; we’d been talking about ions and charged particles earlier in the day. However, I decided on the day that it would just complicate what was a new issue. I’ll probably bring it up this week though as we recurse through Newton’s Laws.