The other day Matt over at Built on Facts had an interesting post, “Pushing Things in Space.” And while it’s rocket science, it’s not really rocket science. It’s Newton’s Third Law in action (and reaction), which is first-semester physics.

There is a lovely scene where the titular robot [Wall-E] uses a fire extinguisher to propel himself through the vacuum of space. There’s no sense in critiquing the physics of a gentle animated film, but it gives us an opportunity to talk about the principal challenge of moving about in space – there’s nothing to push against. On earth you push against the ground with your feet while walking, or with your tires when driving. If you’re in an airplane, the propellers or jet engines pull in still air in front of the plane and push it out the back at high speed. Boats do the same thing with water. It’s just Newton’s laws in action.

Down in the comments, CCPhysicist, aka Dr. Pion, has the real content I want to dissect.

Your remark, “the principal challenge of moving about in space – there’s nothing to push against”, is false. The statement “If you want to push against something, you’ll have to bring it with you” is closer, but still conveys a false concept common in students.

Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion.

I don’t think the statement is false. I think it’s easily misconstrued and is often found to be confusing from the perspective of a beginning student or a reporter. It reminds me of that famous quote that appeared in the New York Times years ago, critiquing Robert Goddard

That Professor Goddard with his ‘chair’ in Clark College and the countenancing of the Smithsonian Institution does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react–to say that would be absurd. Of course, he only seems to lack the knowledge ladled out daily in high schools.

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