How do rockets accelerate in space?

[kiloran]

"As you get further from the Earth its gravity gets weaker." OK

"As you get nearer the Moon its gravity gets stronger." OK

That sounds a bit iffy to me. Gravity (meaning gravitational constant?) stays the same. But the effect of gravity gets weaker as you get further away from the earth/moon/whatever.

--kiloran

[XFool]

"As you get further from the Earth its gravity gets weaker." OK

"As you get nearer the Moon its gravity gets stronger." OK

That sounds a bit iffy to me. Gravity (meaning gravitational constant?) stays the same. But the effect of gravity gets weaker as you get further away from the earth/moon/whatever.

I'm not sure any of that matters - in fact that is what was meant. As a general 'explanation' for weightlessness it's "iffy" enough as it stands!

[kiloran]

"As you get further from the Earth its gravity gets weaker." OK

"As you get nearer the Moon its gravity gets stronger." OK

That sounds a bit iffy to me. Gravity (meaning gravitational constant?) stays the same. But the effect of gravity gets weaker as you get further away from the earth/moon/whatever.

I'm not sure any of that matters - in fact that is what was meant. As a general 'explanation' for weightlessness it's "iffy" enough as it stands!

Sorry, I forgot to enable <pedant mode>

--kiloran

[johnhemming]

Objects that are falling where there is no atmosphere generally are accelerating.

[UncleEbenezer]

Objects that are falling where there is no atmosphere generally are accelerating.

Not in the familiar sense. Bear in mind that every object in space is falling: for example, the Earth is continually falling, most obviously towards the sun, a little bit towards the moon and negligible amounts towards other objects, and also following the sun in its own larger-scale fall. But we don't generally call that acceleration, we call it orbiting.

When you drive your car, your most powerful acceleration is the emergency stop. Unless perhaps you act as a racing driver on the bends.

[XFool]

Objects that are falling where there is no atmosphere generally are accelerating.

Not in the familiar sense. Bear in mind that every object in space is falling: for example, the Earth is continually falling, most obviously towards the sun, a little bit towards the moon and negligible amounts towards other objects, and also following the sun in its own larger-scale fall. But we don't generally call that acceleration, we call it orbiting.

So what do I know? But you would be disagreeing with one Isaac Newton that no "acceleration" (i.e. a change in velocity) occurs in an orbit. I'd say take it up with him, if he were still around...

When you drive your car, your most powerful acceleration is the emergency stop. Unless perhaps you act as a racing driver on the bends.

OK. There is more than one issue involved in all this:

1. Semantics
The word "gravity" is used by different people, in different ways, to mean different things. Also the word "acceleration" is being used in different ways - descriptive vs experiential.

2. Frames of Reference
The same physical situation can be described legitimately in different frames of reference. Without defining the one you are using, confusion reigns.

3. Relativity
I think it's a lot simpler to stick with classical mechanics (and classical relativity) in these descriptions - goodness knows it is confusing enough to a lot of people as it is. (Even one of the Apollo astronauts in a TV interview the other night confessed he could never really come to terms with the details of orbital mechanics and rendezvous - 'To catch up you slow down to go faster so you can catch up...')

[johnhemming]

Objects that are falling where there is no atmosphere generally are accelerating.

Not in the familiar sense.

We are indeed in a world in which meanings are mutable.

If we start out with a definition of the word falling
https://www.dictionary.com/browse/falling

Falling normally refers to the reduction of height relative to Earth, but could be some other planetary body. I personally would not refer to an object in orbit falling unless the orbit was decaying.

Hence my statement in respect of falling is true.

We might as well stick to classical relatively which is easier to understand. However, we should really be able to agree on the usage of words. Objects in orbit are accelerating because their direction of travel is continually changing. The speed is also changing to some extent.

You can think about this in terms of pushing a boat on a frictionless ocean. If you don't apply any force the boat will continue travelling in the same direction. If you paddle sideways you will change the direction of the boat. It will therefore be accelerated even if it does not add to the speed in its original direction. Get the paddling right (and probably slightly against the original direction [vector]) and the boat will travel in a circle. That is a 2 d equivalent of orbiting a planet.

[XFool]

Objects that are falling where there is no atmosphere generally are accelerating.

Not in the familiar sense.

We are indeed in a world in which meanings are mutable.

If we start out with a definition of the word falling

Moving in the direction where time goes slower?

I'll get me coat...

[UncleEbenezer]

Objects that are falling where there is no atmosphere generally are accelerating.

Not in the familiar sense.

We are indeed in a world in which meanings are mutable.

Indeed

If we start out with a definition of the word falling
https://www.dictionary.com/browse/falling

Falling normally refers to the reduction of height relative to Earth, but could be some other planetary body. I personally would not refer to an object in orbit falling unless the orbit was decaying.

Hence my statement in respect of falling is true.

Not quite. You said "no atmosphere", which explicitly detaches you from Earth and puts you in space, where falling is indeed orbiting.

We might as well stick to classical relatively which is easier to understand. However, we should really be able to agree on the usage of words. Objects in orbit are accelerating because their direction of travel is continually changing. The speed is also changing to some extent.

Yep. The moon is constantly accelerating towards Earth, yet (in a geocentric frame of reference, and to a close approximation) returns every 28 days to exactly the same position and velocity.

That could be confusing to a reader whose experience associates the concept with a pedal called the "accelerator" in a car. It could start to look like Escher.

You can think about this in terms of pushing a boat on a frictionless ocean.

You could also just walk or drive around in a circle.

Re: the OP. In the case of a satellite in Earth orbit, to go faster around the Earth you fire the rockets in reverse to slow you down (and of course vice versa). That's because slowing down causes you to fall into a lower orbit, which means you're travelling faster!

[scrumpyjack]

It's all relative.

I recall the story of the Oxford philosophy professor on the train to London, who asked the guard what time Paddington reached the train.

[johnhemming]

Not quite. You said "no atmosphere", which explicitly detaches you from Earth and puts you in space, where falling is indeed orbiting.

If I were in the international space station I would much prefer to orbit than fall.

I did apply to be the UK's astronaut in the 1980s. I still have the t shirt somewhere.

We can have an argument about where the atmosphere ends. The real reason why I said no atmosphere is because if the atmosphere is sufficiently dense it turns momentum into heat.

https://en.wikipedia.org/wiki/Atmosphere_of_Earth

[XFool]

Not quite. You said "no atmosphere", which explicitly detaches you from Earth and puts you in space, where falling is indeed orbiting.

If I were in the international space station I would much prefer to orbit than fall.

Surely, in the ISS (if not elsewhere!), to "orbit" is to fall? It must be the best and simplest explanation of 'zero g' - a term seemingly no longer popular with NASA.

I did apply to be the UK's astronaut in the 1980s. I still have the t shirt somewhere.

Did you ever go on the Vomit Comet? Surely that was falling? What's the difference? (Apart from tangential speed, obviously.)

[XFool]

Did you ever go on the Vomit Comet? Surely that was falling? What's the difference? (Apart from tangential speed, obviously.)

OK, let's compromise. Use the technically better phrase "in free fall" to replace "falling" - with its connotations of "falling down".

[johnhemming]

It is fair to say an object in orbit is in free fall and that it would feel to a human being like falling, but it is not actually falling.

[XFool]

...It depends then on your definition of "falling".

Interestingly, using my somewhat mischievous definition of "falling" a planetary orbit wouldn't be falling as, with a circular orbit, time would be flowing(?) at a constant rate and in an elliptical orbit it would be slowing down or speeding up depending on altitude.

[johnhemming]

...It depends then on your definition of "falling".

inevitably and I gave a link earlier in the thread

https://www.dictionary.com/browse/falling

eg: "to drop or descend under the force of gravity, as to a lower place through loss or lack of support."

[scotia]

The real reason why I said no atmosphere is because if the atmosphere is sufficiently dense it turns momentum into heat.

I'm a bit concerned by that statement. Motion energy can be transformed into heat energy. But momentum of all of the interacting matter remains conserved - i.e. stays constant.

[johnhemming]

The momentum of the falling object. The main thing is that an object that orbits in a dense enough atmosphere stops orbiting and starts falling.

Perhaps one should stick to an increase in entropy being the outcome. In a sense heat is partially the momentum of individual molecules of gas.

[gbjbaanb]

All orbiting things will eventually fall out of the sky (ie the orbit will decay to the point where they hit the ground). In the case of spacecraft, the tiny bits of atmosphere and also the solar wind will keep trying to push it out of orbit - the ISS uses a fair bit of fuel to keep its orbit intact, for example.

But ... how to make something move in space - (apart from expel reaction mass as already described) drop a nuclear bomb out the back and detonate it! Simples! (search for Project Orion)


And everything in orbit is falling, its just that they constantly keep missing.

[johnhemming]

All orbiting things will eventually fall out of the sky (ie the orbit will decay to the point where they hit the ground).

So when do you expect the moon to hit the earth? Or when will Uranus hit the sun.