[alancalverd (OP)]

[This topic was split from here: https://www.thenakedscientists.com/forum/index.php?topic=59801.0]

This raises a different question of equivalence.

Assume a rocket with a very large but finite amount of fuel, accelerating at 1g. The occupant feels as though he is standing on a planet, but knows he is accelerating because his fuel gauge is decreasing.

[Bored chemist]

but knows he is accelerating because his fuel gauge is decreasing.

Unfortunately, the ship was built by  some fool who didn't understand relativity or engines, but thought that having 2 was clearly better than having one.
His aesthetic sense said it would be nice to have them on opposite ends of the ship, pointing in opposite directions.

The occupants of the ship have three choices.
They can fire one rocket- and accelerate, or they can fire the other engine and accelerate in the opposite direction
Those two options use fuel at some rate.

Or they can fire up both engines and use fuel at twice the rate.

And, according to you, that means that with the two engines cancelling each other out and achieving nothing, they accelerate faster.

[alancalverd (OP)]

Most rockets are not designed or piloted by fools.

[Bored chemist]

Most rockets are not designed or piloted by fools.

Does the definition of a fool mean someone who thinks the fuel gauge is an accelerometer?

[alancalverd (OP)]

Most rockets are not designed or piloted by fools.

Does the definition of a fool mean someone who thinks the fuel gauge is an accelerometer?

The definition of a fool is someone who doesn't understand newtonian physics.  If you are burning fuel, you must be producing thrust, so you must be accelerating, unless the rocket was designed by BC. Chacun a son gout.

[Bored chemist]

The definition of a fool is someone who doesn't understand newtonian physics.  If you are burning fuel, you must be producing thrust, so you must be accelerating

And yet I showed how this was not true.

By the way, the Sun is burning mass at a significant rate.
What direction is the corresponding acceleration?

[alancalverd (OP)]

Inwards. You used to place great store by the conservation of momentum. What changed? 

[Bored chemist]

Inwards. You used to place great store by the conservation of momentum. What changed? 

I see; you think that squeezing a vessel is the same as accelerating it...
Must make navigation tricky.

[alancalverd (OP)]

The time differential of momentum is force. Whether it compresses an object or accelerates it depends on whether there is an opposing force. I have naively assumed the mass ejection pattern of the sun to be spherically symmetric but if it isn't, it will behave like a rocket.

I generally navigate with three horizontal forces: thrust, drag, and wind, or four if I'm in a boat. The trick is to direct the vector sum towards my destination (preferably avoiding Heathrow or Dover as appropriate). If you get it wrong, you can meet an opposing force called "terrain", and the vehicle gets compressed.

Rockets are easier as you only need to consider thrust, which is given by F = V dm/dt + pA  where V = exhaust velocity, p = exhaust pressure, A = exhaust area and dm/dt is the fuel burn rate, which you can determine from your fuel gauge.


 

[Bored chemist]

Stop blethering.
The fuel gauge still does not tell you if your ship is accelerating, does it?

[pzkpfw]

The original thought experiment on this specifies there's no window for the occupant of the box to simply look out of to decide what's going on.

The fuel gauge is essentially putting that window back ... providing mundane evidence.

It's a red herring.

[alancalverd (OP)]

Stop blethering.
The fuel gauge still does not tell you if your ship is accelerating, does it?

If it is burning fuel and the laws of physics haven't changed, it must be accelerating. You need to look at the gauge twice, of course, to see if the number has changed, and you need to check the exhaust temperature so that you know whether you are burning fuel in the motor or just leaking it to the cosmos, but that is all part of the umpteen years of training in rocket science that astronauts tend to do while they are waiting to fly. And of course you need to check that your rocket wasn't built by an idiot, like with an engine at each end, but you should have done that before pressing the start button.

Come to think of it, even if you are just leaking raw fuel into  the cosmos, Newton still says you must be accelerating.

[hamdani yusuf]

Come to think of it, even if you are just leaking raw fuel into  the cosmos, Newton still says you must be accelerating.

Perhaps you can try to think out of your silo, and consider alternative mechanisms for acceleration, such as laser propulsion. Don't let technical details obscure your reasoning to find the answer to the basic question being asked.

[alancalverd (OP)]

I can't think of any propulsion system that doesn't consume fuel of some sort, and conserve momentum.

[hamdani yusuf]

I can't think of any propulsion system that doesn't consume fuel of some sort, and conserve momentum.

What do you mean by conserving momentum? Doesn't it mean that it doesn't accelerate? Acceleration is rate of change of momentum.

https://en.m.wikipedia.org/wiki/Laser_propulsion

Laser propulsion is a form of beam-powered propulsion where the energy source is a remote (usually ground-based) laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.

[Bored chemist]

What do you mean by conserving momentum? Doesn't it mean that it doesn't accelerate?

No
A rocket throws stuff out the back, and moves forward.
The total ,momentum of the stuff and the rocket remains the same.

https://en.m.wikipedia.org/wiki/Laser_propulsion
Quote
Laser propulsion is a form of beam-powered propulsion where the energy source is a remote (usually ground-based) laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.

Light carries momentum.

[Bored chemist]

Stop blethering.
The fuel gauge still does not tell you if your ship is accelerating, does it?

If it is burning fuel and the laws of physics haven't changed, it must be accelerating. You need to look at the gauge twice, of course, to see if the number has changed, and you need to check the exhaust temperature so that you know whether you are burning fuel in the motor or just leaking it to the cosmos, but that is all part of the umpteen years of training in rocket science that astronauts tend to do while they are waiting to fly. And of course you need to check that your rocket wasn't built by an idiot, like with an engine at each end, but you should have done that before pressing the start button.

Come to think of it, even if you are just leaking raw fuel into  the cosmos, Newton still says you must be accelerating.

Stop blethering.
The fuel gauge still does not tell you if your ship is accelerating, does it?

If you want to make coffee on board the ship, you have to use fuel.
What effect does that have on the ship's speed?

[Bored chemist]

And of course you need to check that your rocket wasn't built by an idiot, like with an engine at each end, but you should have done that before pressing the start button.

It's quite common to have an engine (of sorts) at each end.
Here's one; it's a rocket plane, but you can clearly see the propeller at the front.
https://en.wikipedia.org/wiki/Messerschmitt_Me_163_Komet#/media/File:Messerschmitt_Me_163B_USAF.jpg

(Read what I said carefully before you comment)

[Halc]

I can't think of any propulsion system that doesn't consume fuel of some sort, and conserve momentum.

If you can define a closed system, then yes, the momentum of that system will be conserved. Most rockets are not considered closed systems.

The definition of a fool is someone who doesn't understand newtonian physics.  If you are burning fuel, you must be producing thrust, so you must be accelerating

Alan, what is your purpose in obfuscating the physics of the equivalence principle? This [was] a mainstream thread and you're pushing very wrong physics in it. Newtonian physics does not conclude that thrust must result from the consumption of fuel, as BC has pointed out.
This is a thread about the EP, and to be equivalent, you must compare apples to apples, and your comparison of apples to oranges only serves to obfuscate. If you want to compare a rocket in space consuming fuel to accelerate at a constant proper 1G, then you need to compare it equivalently. So if you want to compare to a rocket in a vacuum at the surface of Earth consuming fuel to accelerate at a constant proper 1G. The one on Earth will not undergo any coordinate acceleration (as measured by a distant inertial observer) and the occupants of both rockets will be unable to tell via any local test which is which.

I said this as much in my first reply to you, which I notice you ignored. Why do your posts belong here? Why shouldn't I move all this nonsense to the lighter side?

BC, you're allowing yourself to be led by the nose by Alan. Your post about certain vehicles with propulsion at one end or the other is entirely irrelevant to the equivalence principle. If you want to take apart Alan's assertions, keep it to the parts that are relevant to the topic.

[alancalverd (OP)]

I hesitate to derail the discussion further, but If you burn fuel in a rocket engine, do you generate thrust or not? Are you suggesting that the moon landings were faked? They did involve an awful lot of fuel being burned, and as far as I can see, a lot of momentum transferred thereby to the payload. 

Reply #73 resolved my paradox very neatly and politely.

This leaves us with the observation that the sensation of doing nothing on the surface of a planet is indistinguishable from the sensation of burning fuel at an ever-increasing rate in the absence of a large external mass. From which we derive relativity.