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.
The writer of the editorial was making a very fundamental mistake: thinking that in order to accelerate, one must have something to push against, which means that the object then pushes back on you. And if there’s nothing there to push back, then changing your state of motion is impossible. Which is, of course, crap, because that’s not the principle behind rocket motion, and is one more anecdote in support of not getting your science from the editorial page of a newspaper. And what Matt was pointing out is that this is just a subset of ways to make you accelerate. What you need to look at is conservation of momentum.
Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion.
Yes! Exactly. But you can’t have one without the other; they come in pairs — A exerts a force on B, while B exerts an equal but opposite force on A, and it is the force exerted on an object that causes it to accelerate. This is a common stumbling block of students, but it’s not an error Matt is making here.
OK, on to the next tidbit:
It doesn’t take any force at all to move or keep moving. It only takes a net force to change your motion. That is the number one misconception people got from watching Star Trek.
This was one of my gripes on many episodes. It was the plot device of making the ending suspenseful because nobody onboard the ship understood basic physics. No, you don’t have to reprogram the isolinear chips to change the warp coil configuration to unframmenstanz the glopulator and deliver the high-energy tractor-beam signal all at once. A force one-tenth the size but exerted for the whole episode would have done just as well, but that makes for boring TV and leave you short of your cliché quota. (e.g. fewer opportunities to say, “Make it so.” Or in the episode with the transdimensional hypergalactic loom, “Make it sew.”)
Down at the end there was a remark in response to my comment about using photons for propulsion
Hence Tom’s point about photons, although I would not want to fly behind a laser powered aircraft or compute the efficiency of its “engine”. Since p = E/c, you might benefit from non-laser x-ray propulsion, but I wouldn’t want to figure that efficiency or be behind it either.
One of the interesting things about photon propulsion that’s not readily apparent from this is that for a given amount of power, the force you get is independent of the wavelength — there would be no advantage to using X-rays. The photon momentum is E/c, so you get the same momentum from one X-ray photon at 2 keV as you would from a thousand orange (621 nm) photons at 2 eV, which means you are free to choose the most efficient photon source. The problem is that the momentum is really small. A while back I confirmed a calculation that to levitate a 1 kg object you need about 1.21 Gigawatts, assuming perfect reflection, so getting this 1g acceleration in a propulsion system would require twice that. For less Back-to-the-Future-esque values of power (i.e. something reasonable), your acceleration will be somewhat . . . restrained.
Update: Matt follows up on his post