Genady

OK, one more time. They start from dP/dt=0. Then they rewrite this equation in such a way that the characteristics of the rocket are separate from the characteristics of the ejection. This allows them to express force on the rocket separately from the force on the ejection. The end. They don't need and don't apply F=ma anywhere, and they don't need dP/dt separately for the rocket. If you don't get this, I can't help you anymore.

That is how it should be applied.

Saying that the units match the units of force is not the same as applying F=ma. It is only your misinterpretation of their derivation, that they apply F=ma.

No, this is not. They don't apply anything else to get the thrust. They already got it from dP/dt=0.

No, they don't apply F=ma to say it. They take the result of previous calculation which they got from F=dP/dt, and call a component of that result thrust. It is a force, but they did not derive it from F=ma, but from F=dP/dt.

Right. And then they say, "Because there are no external forces, dP/dt=0." That is, dP/dt = external forces.

Yes, they do. They start the derivation with calculating dP/dt by expressing delta-P/delta-t and then taking delta-t to dt; see 1.3 and after it. Then they separate two parts of the dP/dt to get the equation 1.5.

From your link:

Is it an opinion poll? My answer: the rest of the universe.

It is not my original idea. It took me some time to find where I got it from, but here it is: Quantum Field Theory for the Gifted Amateur, Tom Lancaster and Stephen J. Blundell, 2014, Oxford University Press

It is perfectly correct, but I have a caveat to add. I think that all photons are virtual. We never observe photons directly, only their interactions with some detectors. The so-called real photons are just virtual photons that last so long that their deviation from the shell is immeasurably small.

They will not match in the case of Earth, too, if length is in cm, or length in feet and mass in pounds, etc. (in case the OP does not realize what mismatched units mean.)

Right, and that was my simple comment above:

1 AU = 1.5x1011 m g/G = 9.8 / (6.67x10-11) = 1.47x1011 kg/m2 Close... 😉

In that case, the person will need to check in the list of facts I believe in, if that fact is there: Item 1a: My name is Genady. Item 1b: I am a member in SFn. Item 1c: I live in Bonaire. ... This list is longer, but finite anyway. Alternatively, the test could go like this, for example: They: Do you believe it is a fact that an intelligent being created Universe? I (quickly checking my list): No.

I don't need to know, because I am not the one who asks the question. I am an object in this test. The subject, the one who wants to know if I believe in God, supposedly has some idea what it is. Then they go through my list: Item 1: facts and logic. Is it God? - No. Item 2: Earth is not flat. Is it God? - No. Item 3: Money helps. Is it God? - No. ... etc.

g/G has units of mass per area. 1 AU is a distance. How can they be equal?

They are both plausible definitions, but the second one has a problem, IMO, that it requires some definition of God, and this is problematic. The first definition, OTOH, does not require it and thus is easily tested. The second definition leads to farther confusions about 'which God don't you believe in?', 'does it include Buddha?', 'is Jesus God?', 'can you prove that God does not exist?', etc. OTOH, the first definition only needs to look at the list of things I believe in - which is not very long and definitely finite - and to check that there is no God on this list.

Of course, they don't. That's why we tried to point out the wrong assumptions in your calculations which led to the wrong conclusion that they do.

I wish. +1

I'm starting to think your arguments are less than logical.

This is even worse. Now you decide for other people what their opinion should've been. I show that your argument is illogical. Of course I don't argue it, because there is no way to logically argue an illogical argument. Here is another example of how illogical your argument is. There, you talked about atheists. Now, you replace it with the attacks on religion.

Only if you stick with the second definition.

Exactly. As for me, for the most of my life I thought that the first definition is the accepted one. Only discussions in this forum convinced me that most members rather relate to the second one. Since then, I don't call myself an atheist, at least not in this forum.

Variable substitution is not a logical fallacy. Your downvote of my post is a bad mark on you.