A Conversation for Gravity

Errors

Post 1

Hoovooloo

Spacecraft use the force generated by their booster exhaust to circumvent the gravitational force that ordinarily keeps them earthbound. Later they use the centrifugal force due to their orbit and stay 'up there', circling the Earth like other man-made satellites.

er... wrong, I think. Spacecraft don't use the force generated by their booster exhaust. That's the principle of operation of a jet, and only works inside an atmosphere where the exhaust has something to push against. Also, there is no centrifugal force holding anything up in orbit. In fact, only gravity holds something in orbit and prevents it from continuing in a straight line forever.

Think of it this way. Throw a stone away from yourself. Eventually, it falls back to the earth a certain distance away. Pretty obvious, because locally (i.e. for a few miles around any particular point), the earth is basically flat (please don't mention mountains etc). But if you throw it really hard, it could go over the horizon before it lands. This is because at that sort of scale, the curvature of the earth starts to come into play.

Now throw it REALLY hard. Exactly as before, it will go up, then start to come down. However, if you throw it *just* hard enough, its path back to earth will be parallel to the surface. Put another way, it goes up, but when it starts to fall down, it never gets there! It just keeps falling, around the earth. The only forces involved are the initial push to get it there, and gravity, which keeps it there. No centrifugal force required.

Please correct this, it is *such* a common misconception and it's a shame to see the Guide (especially the Edited Guide) perpetuate it.


Errors

Post 2

NexusSeven

I'll direct the Editorial team's attention to this one on Monday. smiley - smiley

Cheers,
N7 (English graduate, and my how it shows! smiley - winkeye


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Post 3

Mycroft

Jets do work in space - the exhaust pushes against itself.


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Post 4

Hoovooloo

No, jets do NOT work in space (i.e. in a vacuum). To simulate the action of a rocket in space, sit on a playground swing and hold something heavy, like a medecine ball. Throw the ball away from yourself. You'll start swinging, because the action of throwing the ball had an equal and opposite reaction - you moved too, in the opposite direction from the ball. This is the principle of operation of a rocket.
Now imagine doing the same thing with two such balls in quick succession, in such a way that the second hits the first. The initial impulse from throwing is the same each time. The extra push you get from the fact the second ball has something to push against is - zero.
A jet works by locally accelerating the medium through which the vehicle is travelling (air in the case of a plane or water in the case of say a squid). In space, there is no medium, and the exhaust from the rocket engines is certainly far too diffuse to form enough of a medium to "push against".
The fact that someone believes jets work in space strongly enough to actually write a reply about only goes to show that this entry needs correcting, preferably by a physics graduate, as soon as possible.


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Post 5

Dr Hell

Of course jets work in vacuum. (Try it out with any ordinary office chair). If you throw matter away a great speed the center of mass will stay the same, for that you will have to move FROM that center of mass...

Someone did not understand basic physics here.

HELL

(in need I can write more tomorrow)


Errors

Post 6

Dr Hell

AND: The sattelites DO stay up there - in that we should agree. And they ARE subject to the gravitational force (Earth is made out of mass - a lot to be precise, and the sattelite - too - is made out of matter.) So if they ARE constantly being ACCELERATED (by gravity) what would YOU call the force holding them up there (knowing it is opposing the gravitational force which is pointing directly to the center of the earth which by the way is the center of the orbit of the satellite)? I would call it centrifugal. Your Medicineball example: You GET extra push - YOU say that. In space movement is conserved (you do not have to be constantly accelerated to compensate stuff like friction) so the extra push is enough to propell you in space (or change directions or decrease your relative speed - whatever - in fact that IS how maneuvers ARE ACTUALLY MADE IN SP├ťACE!!!!. What happens to the medicineballs is totally irrelevant (for the guy in the spacecraft anyway).

sincerely yours,

HELL

PS: Or I must be going totally nuts...


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Post 7

Hoovooloo

Fascinating. You seem to consider yourself an expert on satellites, and yet you can't even spell the word satellite.


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Post 8

Hoovooloo

OK, the above was a cheap shot. But you really have got everything here completely wrong. You really need to speak to someone who actually understands physics if the explanations I've already provided aren't clear enough. I'm going to try again though.
Firstly, "Of course jets work in a vacuum. Try it out with an office chair." This is a startlingly silly thing to say. Jets DO NOT work in a vacuum. Rockets DO work in a vacuum. All the guff about centres of gravity (which doesn't really make much sense I'm afraid) doesn't change this fact. In fact, I think you're talking about rockets anyway without even realising it.
Secondly, yes, satellites (one "t", two "l"'s) do stay up, duh. As explained in painful detail in the previous post, NO FORCE is required to "keep them up". Physicists have known perfectly well since the time of Newton that if a body is in uniform motion in a straight line, it will continue in that straight line forever unless acted on by some external force.
The force acting on a satellite is acting INWARDS (i.e. "downwards"), towards the earth, preventing it from continuing in a straight line. There is NO FORCE acting to "keep it up" - just its tendency to continue in a straight line.
Take a conker on a string and spin it round your head. Is the string pushing the conker out? Or is it holding it in, preventing it from simply continuing in a straight line?
"Centrifugal force" is a useful shorthand for something quite complex, in much the same way that the "miniature solar system" model of the atom is a useful way of explaining things, right up to the point where you have to explain that it's a complete fiction that you invented to make things easier to understand. Centrifugal force does not exist any more than atoms are like little solar systems.

For more information on the difference between a rocket and a jet, see my Guide entry. http://www.bbc.co.uk/h2g2/guide/A574959


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Post 9

Mycroft

Hoovooloo, jets and jet engines are not the same thing. If you think otherwise then feel free to take it up with JPL.


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Post 10

Hoovooloo

Ah, technical jargon. Jets and jet engines not the same thing. Stop someone in the street tomorrow and ask them what a jet is. Make sure it isn't a jeweller, because they'll probably start on about a hard black variety of lignite. Them aside, if you can find even a single person who doesn't in some way refer you to gas turbine driven aircraft, whose engines are the matter at issue here, I'll be extremely surprised. Yes, a jet is not necessarily a jet engine. I even concede that a rocket engine expels a jet of gas from its exhaust. The point is that what 99.9% of the population think of as a "jet" operates according to one principle (inducting and accelerating a medium using fuel combustion), and what 99.9% of the population thinks of as a "rocket" operates according to a quite different principle (accelerating gas purely from onboard combustion with no need for any local medium).
If you think otherwise, feel free to give NASA a ring and tell them jets are cheaper than rockets, and why don't they fit them to the space shuttle. NASA's phone number is (713) 483-3111.


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Post 11

Mycroft

I've no need to tell NASA that jets are cheaper than rockets: they already know. Jets are those things they fitted to the Space Shuttle so it could be maneuvered without all that tedious combustion business and function in a remarkably similar way to the squid you mentioned earlier. I wouldn't want to steal your thunder by pointing out it doesn't work, so perhaps you should call them - you've already got the number.


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Post 12

Dr Hell

Excuse me for my poor English mister... But there are many languages being spoke around the world and I did not have enough time to concentrate on my English -- which in my view suffices to communicate quite well, at least when talking to people with a minimum of tolerance and understanding.

Let me start with a question: Ever heard about actio & reactio? Ever heard about centripetal force then? If a satellite (spelled it correctly now?) is not moving in a straight line, then it is subject to a force. OK. I will try to concentrate... Let us start from scratch. So the sattelite's trajectory IS being bent. IF you suggest that the sattelite is being PULLED by gravity and constantly MISSING the EARTH and that is why it remains up there... FINE, you are talking about the centripetal force then. If you suggest that the sattelite is constantly FLEEING gravity, which keeps him bound to his orbit... FINE TOO, You are talking about the centrifugal force.
Either way it is correct (we know that since Newtons actio et reactio law.)

Oh BTW... Are you a troll? Are we feeding you? Or did you REALLY not understand this?

HELL


Errors

Post 13

Dr Hell

In the entry I meant the booster exhaust. And not a jet engine, or a jet or whatever. That part of the discussion arose here in the thread...


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Post 14

Dr Hell

There you go... The entry text:

"Spacecraft use the force generated by their booster exhaust to circumvent the gravitational force that ordinarily keeps them earthbound. Later they use the centrifugal force due to their orbit and stay 'up there', circling the Earth like other man-made satellites."

Hooloovoo's intervention (excerpts):

"er... wrong, I think. Spacecraft don't use the force generated by their booster exhaust."
--Ever watched a Shuttle launch on TV? What would you call that fire comming out of the bottom of that white thingey climbing the sky? I would call it the booster exhaust, that is being used to lift the shuttle.

"[...]That's the principle of operation of a jet[...]"
--I do not think so.

"Also, there is no centrifugal force holding anything up in orbit. In fact, only gravity holds something in orbit and prevents it from continuing in a straight line forever."
--See above.

I am sorry if I sometimes overreacted. But Hooloovoo was certainly not polite in any of his mails. Maybe I have formulated things in an unorthodox way:

(a) I am not an english speaking person
(b) This entry has been edited and peer reviewed.

I would never insult a person if he/she had written something he/she misunderstood or did not formulate well. I DO think I have a good understanding of physics and I do think that if someone has misunderstood something here then it was Hooloovoo -- which makes his attacks even more pathetic.


Errors

Post 15

Dr Hell

Hooloovoo's misunderstanding:

From the article he might have gotten the impression that Spacecraft are USING the booster exhaust while in ORBIT.

NO. I did not SAY that. I said they need the exhaust to get them there.

Once they are up there, they do not need any further acceleration.(The boosters are actually jettisoned - as we know)

So they will not work like a jet or anything else.

Hooloovoo's unpoliteness made me understand HIS point wrong too.

Let us smoke the peace-pipe... unless there is anything else to discuss.

Salve,

HELL


Errors

Post 16

Orcus

Sorry, impolite or not I'm going to have to agree with Hooloovoo on this one. I was taught at about the age of 14 that centrifugal force does not really exist.

Any physicists out there, I think we need one to straighten this out?


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Post 17

Dr Hell

I AM a physicist - ******** - centrifugal force and centripetal force call 'em what you want It is all the same.

GRUMBLE.

HELL


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Post 18

Orcus

You told me you were a chemist smiley - winkeye

I don't particularly want to get heated about this - almost the first thing they taught us when we did the laws of motion at school was that centrifugal force - a force pushing you "outwards" when you are on a roundabout for exmple does not really exist. Centripetal force, which pulls you in certainly does (ie the force applied by your arms to prevent you from flying off at a tangent as said above).

If you simply want to rename the centripetal force as centrifugla force then yes they are the same thing but surely by the above they are opposite and therefore different.

I guess this really depends on what people's physics teachers said.


Errors

Post 19

Dr Hell

I am a physico-chemist...

And what got me heated is the TONE used by Mr.Hoo... He could have pointed this all out without having to get personal.

You Orcus are a chemist. You probably have used a centrifuge at least once in your life. To understand a force it is completely irrelevant which part is doing the ACTIO or which part is doing the REACTIO bit. Both are there and totally equivalent.

So if ANY teacher ever told you that a centrifugal force does not exist, and you prefer to believe HIM go ahead. All I am saying is that it does not matter: forces always come in pairs.

OK... The sign is different. You cannot JUST replace them words without inverting the sign.

Cooling down,

HELL


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Post 20

Orcus

I guessed you were a phyisical-chemist, that's why I put the smiley - winkeye there smiley - smiley

We are really just getting into semantics now. You've mentioned cetrifugal force and have at least three other scientists gunning for it. Does that not say something. All I'll say is that I completely agree on what Gnomon said in the Peer Review thread.

Centfigues? smiley - yikes too many times.

I don't blame you for getting upset above, it was pretty agressive up there.

So what do you actually study anyway - are you a PhD student or have you passed that stage?


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