hmmm

You're welcome.

As STS60 pointed out, pressure change isn't the central issue. In this case, a mass of air is being thrown in one direction, so (if you're holding the air tank) you'd be sent in the other direction. Physically, it doesn't matter why the air is being thrown in one direction, or even that it is air: It is simply the fact that something is being thrown in one direction, sending you in the other.

As for how much it takes, that depends on what you're trying to do. It doesn't take more just because you're in a vacuum, and in fact, sometimes fairly simple gas rockets are used for maneuvering in space.

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The Leif Ericson Cruiser

No. If you're sitting in your wheeliearmchair, and want to move without pushing off anything else, you have to expel something from you at speed, and you'll move in the opposite direction. The example given was for you to hold a tank of compressed air and release the valve. This would work whether you were in your office or floating in space.

Another way of looking at the idea of Newton's law about equal and opposite actions is to imagine yourself on ice skates on an ice rink. Throw a ball as hard as you can without falling over. What will you do? Stay where you are? No, you'll slide slowly in the direction opposite to that in which you threw the ball. Same principle.

No, it would take less, because of friction. Think of your wheeliearmchair again. Which surface is easiest to move over? Thick shag pile carpet? Flat carpet? Or a tiled floor?

In the case of a vacuum, it offers the least resistance to movement, because there's no stuff to offer any friction.

Friction - resistance to being moved - is different from force. I think you may be confusing the two.

As for how much force is needed to move, any force will move any mass. If the force is small and the object is large, the object will move slowly. But if the force is large and the object is small, the object will move quickly. Think of how hard you can throw a shot put, and how hard you can throw a tennis ball.

While we're practicing thread necromancy...

Isn't this a false dilemma? Surely there are other options. For example: The Iraqi airplanes became unusable after their exodus anyway. In effect they were neutralized without any need to put Tomcats in harm's way. Who's to say this wasn't understood by US commanders at the time, and that the outcome isn't a reflection of their foresight and wisdom?

The Third of Newton is hard to wrap your mind around because many of the examples you think are describing the law are really just describing ordinary pushing. It's convenient to try to think of a rocket as pushing against something by means of its exhaust. But a better way of thinking about it (informally, anyways) is the rocket pushing against its exhaust. Now that's really not physically accurate, but helps if you think of the thrust chamber as trying to separate -- or increase the distance between -- the rocket and its exhaust.

If you stand between two shopping carts (again, well oiled), one filled with soup cans and the other filled with bathroom tissue, and you push them apart, the one with the cans doesn't go as easily as the one with bathroom tissue. That gives you a sort of practical notion of the relationship between force, mass, and acceleration.

Now think of Sts60's experiment again. Sit in your office chair (well oiled) on smooth hard flooring and toss a bag of rocks, or some books. You'll move. But why? Is it because the thing you threw pushed against the air? No, it's because you pushed against the books. The books responded by going away from you fast; but you responded also by moving away from the books -- albeit slower because you have more mass.

That's why we talk of conservation of momentum. The combination of velocity and mass is momentum, and it's that number that is equal in both directions. The momentum of the books one way is equal to the momentum of you the other way. Momentum is said to be "conserved" because the velocities that make up the momentum are in opposite directions. We consider them positive and negative numbers respectively, so the net sum is zero -- conservation.

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Clavius Moon Base

Welcome dpstdd.

Instead of trying to give other analogies I'll just ask a simple question:

Ever since the space age began, rockets have been used in space to propel satellites, manned or otherwise. That rather suggests that they work without air, doesn't it?

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"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

I don't think dpstdd is here necessarily to argue that rocket propulsion doesn't work without something to push against. I think he's trying to reconcile the fact that rockets do work with what he misconceived the governing principles to be. A lot of people have the same misconception, so it's good of him to ask.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

TOPIC OF THE TIMES

"...After the rocket quits our air and really starts on its longer journey [to the moon], its flight would be neither accelerated nor maintained by the [proposed by Goddard solid rocket based on] explosion of the charges... To claim that it would be is to deny a fundamental law of dynamics, and only Dr. Einstein and his chosen dozen, so few and fit, are licensed to do that.

"...That Professor Goddard with this 'chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action and 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..."

Editorial comments, The New York Times, 13 January 1920.

Maybe I'm just too cynical. The resurrection of an old thread in the conspiracy theories section to ask a question that could have gone in 'questions and answers', the start of the question with 'if there is a vacuum round the Moon', and the use of 'gotcha' in one of his replies just set some alarm bells ringing in my head.

Isn't a rocket engine just like a series of explosions? As each explosion occurs the gases expand outsards from the point at which they origniated. In one direction they happen to impact the inside of the rocket engine, so they impart a bit of momentum. Get them frequently and powerful enough and you start to shove your rocket along.

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"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

I've always understood it as follows.

A rocket is basically a ball with a hole it one end. As the chemical reaction occurs, the molecules press outward everywhere on the inside of the ball, except, of course, where the hole is.

This causes an unbalanced (net) force on the opposite side, hence acceleration.

Is that a decent explanation? (Although conservation of momentum works for me as well)

Pete

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PJE

There's so much I don't know about astrophysics. I wish I had read that book by that wheelchair guy.

An interesting example was the venting in the Apollo 13 situation. I forget the exact source, maybe a thremal system outgassing, but the thrust was seemingly negligible. It was enough to change the earthbound trajectory to the point of requiring an additional midcourse correction, or two.

My understanding fits best with Jay's "Toss a heavy rock from a wheeled chair" analogy. By causing a violent reaction, you are basically speeding up your accelerant (fuel). But since it can only escape in one direction (i.e., the exhaust opening), you are in effect "throwing" your fuel.

Nobody seems to have a problem visualizing a gun firing in space, in that the bullet would exit the barrel but it would also propel the gun in the opposite direction. The only difference in the rocket is that the combusted fuel is not solid like a bullet...but it still has mass and momentum.

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"A long time ago, yet somehow in the future"

I think that is the biggest issue. I look back to when I didn't understand the concept. I knew Newton's laws, but how can it apply to smoke? Smoke has no weight. It was hard to imagine the volume that is being presented.

Expanding gases made more sense at the time as... they take more room when they expand...and something has to give. Kind of like displacement.

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Of course, they did print a retraction of that in, um, July of '69.

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"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"

"You can't erase icing."

"I can't believe it doesn't work! I found it on the internet, man!"

Newtons First Law.

Please take a hard science course next semester.

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Is that a decent explanation?

It's a decent explanation of pressure thrust, which describes the thrust component derived from the net static pressure of the exhaust gas as it leaves the thrust chamber. This can account for a substantial percentage of the total thrust an engine produces in a vacuum.

But it doesn't capture the mechanism of momentum thrust.

In the ball-and-hole model, the direction in which gases exit the hole wouldn't matter. They could go straight out the back, or they could fan out as intuition suggests they would naturally do. But in conservation of momentum, we find that direction does matter. Momentum is a vector quantity because velocity is a vector quantity. The more particles that depart directly along the axis of motion, the greater the momentum reaction in the rocket. A rocket exhaust that departs in a neat column causes the strongest conservation-of-momentum reaction in the rocket (which is constrained by its solid nature to make all its particles go the same direction).

Thence nozzles. By fitting the hole in the ball with a nozzle that directs fluid flow into a column, you ensure that the departing particles stay in a column as much as possible, keeping their velocity vectors reasonably aligned and setting up the cumulative momentum better. Under equivalent mass-flow conditions, the nozzled ball will perform better. That makes the unbalanced-reaction model incomplete.

Keep in mind also that there are engine designs that throw mass by means other than generating fluid pressure. The goal in any Newtonian engine is to throw mass very fast. A common way to do that is create a large amount of pressure in a fluid and limit how it can escape; in turn accomplished by heating it through an exothermic chemical reaction and allowing it to expand, or by catalyzing a decomposition reaction that results in greater volume. But ion engines throw mass by accelerating it through magnetic fields. There is no unbalanced reaction -- no "ball with a hole" -- just pure momentum.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

Oh, yes; a masterfully dry retraction:

"Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error."

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Makes sense to me, especially since I've taught conservation of momentum at the senior highschool level (I also teach vector algebra as well)

So you're saying that in a non-ion engine, that the the thrust is from both momentum and pressure, but mostly momentum due to efficiency of design?

Pete

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PJE

There's so much I don't know about astrophysics. I wish I had read that book by that wheelchair guy.