There's no such thing as centrifugal force. There' s no force pushing objects beyond geosync, there's just insufficient gravity beyond that point to keep them in geosync.

David

To stay in orbit an object needs to maintain a particular speed. Too slow and it starts to move towards earth in ever decreasing orbits. Too fast & it shoots off in to space.

In other words, an object in orbit is still being pulled down by gravity, its just moving fast enough forward to miss the earth as it "falls".

David

Posted By: davidfreeboroughan object needs to maintain a particular speed. Too slow and it starts to move towards earth in ever decreasing orbits. Too fast & it shoots off in to space.

Yeah but who says what its speed is? relative to what? what if earth was a homogenous ball where every part of it was same as any other part of it, with no visible surface features, or even distinguishing objects within, how would the object know it was moving relative to that uniform sphere? Certainly not visually or other electromagnetically. And what non-material part of earth would be able to say whether the object was whizzing round it or standing still?

Posted By: fostertom...how would the object know it was moving relative to that uniform sphere?

Tom, remember those cartoons you saw as a kid where the coyote or whatever ran off the cliff but didn't fall until he looked down and realised his problem? I hate to disillusion you but that's not how the real world works.

PM me if you feel you're still confused on the Santa issue.

Posted By: davidfreeboroughToo slow and it starts to move towards earth in ever decreasing orbits.

In general if the satellite is going too slowly for a circular orbit at its current altitude it'll just be in an elliptical orbit. That won't decrease unless it goes through the atmosphere causing aerodynamic braking (or it goes through the planet causing lithobraking) or there are other minor forces acting on the satellite to take energy out of its orbit.

Posted By: SeretAs for things in orbit, (ignoring the motion of the stars) something could tell it was orbiting a featureless ball by the simple fact that it isn't getting any closer

Is that what Einstein says? Seems a circular argument - no pun intended! Postulate that A (orbiting) causes B (centrifugal balance re gravity), so when B happens it proves it's A causing it.

So things needn't 'know' they're moving - they just are. Who says? How can you say whether a satellite is moving relative to earth? Leave out visual contact or

Posted By: davidfreeboroughground based radio navigation beacons

Both those imply that the criterion is movement through the electromagnetic field - likewise leave out movement through earth's magnetic field. When there is absolutely no physical contact, not even highly attenuated atmosphere, between satellite and earth, what exactly is the shared field within which relative movement is happening? Sounds like late-Victorian Aether - an invisible fixed xyz matrix within which all objects move. I don't think so.

Posted By: davidfreeboroughinertial navigation systems which integrate acceleration in 3 planes

is more interesting.

Posted By: fostertom
How can you say whether a satellite is moving relative to earth?

Some of them aren't.

Take a geostationary orbit. Relative to a point on the Earth looking up at that satellite, it doesn't move. It just hangs there in the sky perfectly stationary, tons of steel and aluminium perched effortlessly above your head as if by magic. However, step back a bit and you realise it only appears to be static because it's actually orbiting the Earth at the same velocity that you (being nailed to the Earth) are spinning. And the really weird bit is that even though it takes the same amount of energy to move north and south across the Earth as it does to move east and west, that speed you're moving at (and therefore the amount of energy it takes to hang a TV satellite weightlessly above you) differs hugely. That's why equatorial sites for launching rockets are so highly prized, as the rocket is going a lot faster even when it's sitting on the pad.

All speed is relative. There are no fixed reference points, therefore everything is moving. I'll say that again: everything is moving. However, when things are moving at the same velocity, they might not appear to be moving relative to each other. In a way the idea of anything being stationary is a kind of personal conceit caused by your restricted view of the universe.

Posted By: SteamyTeaIf sometime is orbiting the Earth, isn't it constantly accelerating?

Elliptical orbits will be as their distance from the body they're orbiting changes so they'll experience a constantly changing acceleration due to gravity. A perfectly circular orbit won't. In practice small forces such as atmospheric drag, gravitational perturbations from other bodies and even solar wind mean you need to inject energy occasionally for station keeping.

Posted By: SteamyTea

Posted By: SeretA perfectly circular orbit won't

Even then it is still accelerating as the direction is changing:

Well, yes, if you're going to break it down into x and y, but that's hardly a unique property of an orbit. It's speed is constant, which is what I assumed you meant. There will be no acceleration measured by the satellite (for example by an inertial navigation system) itself in such an orbit, even if your chosen coordinate system decides there is one.

No one's answering my question

Posted By: fostertomHow can you say whether a satellite is moving relative to earth? Leave out visual contact or ....

It's got to be a universal field. I was hoping someone wd say something so I cd really understand that. I think it took Einstein a while.

This very question arose for me in the middle of my O-level Physics exam. I just wrote and wrote about it as it came to me, answered no other questions, got an excellent mark! They must have thought I showed potential. Sadly, got sidelined into architecture - and amateur debates like this.

Tom, has no one ever told you the truth?

No one gets "sidelined into architecture", they're steered there as a refuge from the realities of the real world.

Sorry to be the one to break the news, mate. Honest.

Posted By: SeretAcceleration can be detected by accelerometers

but acceleration relative to what? What if you're a zillion light years away from any other object - what are you then accelerating relative to? "space"?

Posted By: Seretaccelerometers ... don't require measurement relative to another body

That's the rub - please explain!

Posted By: fostertom

Posted By: SeretAcceleration can be detected by accelerometers

but acceleration relative to what? What if you're a zillion light years away from any other object - what are you then accelerating relative to? "space"?

Posted By: Seretaccelerometers ... don't require measurement relative to another body

That's the rub - please explain!

Any acceleration has to overcome inertia, that can be detected by an accelerometer.

Imagine you're locked in a windowless box sitting on top of a bunch marbles. Sitting in your lap is a tray with another marble in it. If someone shoves your box from outside you can detect that by noting the fact that your marble moves on it's tray*. That's a very basic accelerometer. Even though you've got no external reference you can tell that something has pushed you, and you know by how much and in what direction.

That's the basis of inertial navigation, and it's how things like submarines can know their position and orientation without anything to see and without radiating anything detectable to find their position. It's also used extensively in aircraft and spacecraft.



*Well, in reality you can detect it because you've got a lovely 3-axis accelerometer built into your ears.

Is this digression towards accelerating a mass over a distance anything to do with picking up a virus on the laptop you've left charging overnight and which you're thinking about smashing against the wall?

You've got to blame something!

Picked up the "Police Central e-crime Unit" malware thing on Thursday, which accused me of viewing kiddy-porn and other extreme 'sports' involving domestic and farmyard animals. Designed, of course, to put the fear of christ up you. All I'd been doing was trawling through various turbine-related sites for technical info on the Enercon machine proposed for the installation near here. Took me a few seconds to realise what I was looking at and then, when I did realise I did indeed nearly throw this thing against the wall.

It just makes you so mad at the things that bring so much crap into your otherwise mundane, perfectly law-abiding life.

Yeah, I know, should have bought an Apple Mac, but I can't even afford to buy the lottery ticket that would finance that purchase, so will have to continue making-do with Windows.

Luckily I've had that particular piece of malware before and so knew to just power up in safe mode and restore to a previously saved restore point.

I did go back to the last two sites I'd visited immediately before the Met logo came up with its dire warning to pervs like me to pay up their £100 "fine" or find myself dragged through the courts and thrown into prison, but it didn't happen again. If it had I'd have warned those sites that they'd been hacked.

Last one I had was years ago when I was on the phone helping my son's partner find her way through various sites looking for a wood-fuelled range cooker. Had no idea I was engaged in an erotic activity but probably explains my aversion to wood-burning stoves.

Have a look at http://www.wired.com/gadgetlab/2012/08/apple-amazon-mat-honan-hacking/all/. It might change your view of Apple's security. (A journalist had all has data on all his Mac gadgets wiped by a hacker or 2.)