Genady

It means to me that F=ma applies to momentary value of a if a changes in time.

Yes, what you say is GR. What I replied to above, is not.

I can't believe you don't get this simple question, but I try again. You have acceleration changing in time, you observe it, you get the values as they change. Mass is known. You need to find force. One equation, one unknown.

But you need to put some number for a to find F. Which number will you put if a is changing?

Let's say you observe a body moving with a variable acceleration and you want to know a force.

I see. Sorry, but it is not how GR works.

My question is, what value of a should be used in F=ma if a is changing in time?

OK. How it works if the acceleration changes, for example, continuously increases?

I doubt it says that the law is not applicable if mass changes. Does it apply for constant a only?

Could you show me what exactly that physics text says?

It applies to a momentary mass, a mass at a moment in time.

I mean misunderstanding. I have four in total. But everyday language is English.

I see. When you said, 'considered for' you meant, 'relative to'. OK. Lost in translation.

Yes.

The rocket is not an inertial frame because it accelerates. Consider relative to the ground.

Constant velocity relative to the rocket, but it changes relative to an inertial frame together with a changing velocity v of the rocket. Check in your source, how that ve is defined.

In GR, free falling object considered to have a locally inertial reference frame. The things float freely inside that frame, i.e., relative to the free falling object. Relative to the outside observer who is stationary relative to the central mass, the free falling object accelerates.

Without accelerating? It will accelerate. How is it related to the space being stationary? Perhaps, we mean different things.

An example could be the spacetime around non-rotating spherically symmetrical massive body, aka Schwarzschild metric. The space there is stationary, and the spacetime is curved, isn't it?

From the event of the ship leaving to the event of the ship returning, everyone on Earth experiences 10 years, and everyone on the ship experiences 1 week. What is confusing about it?

Sorry, when I said, "I think that OP rather is about a centripetal acceleration", I meant OP in this thread we are here now, namely, Willem, https://www.scienceforums.net/topic/128865-the-earth-is-not-accelerating-upwards/

And the clock/calendar in the ship will indeed show 1 week.

What is wrong about it?

I can't come up with any caveat to add here .

But it is precisely what it means.