sophiecentaur said:
????
Sorry but I just don't get that.
1. A gravitational force is experienced by an object on the floor of the ship and the object gains KE.
No. In the frame of the ship, the object on the floor is just sitting there. It gains no KE.
It is subject to an interaction force from the floor of the ship. But that force is applied over zero displacement, so it does zero work. It is subject to the fictitious force from the acceleration of the ship. But that force is also applied over zero displacement so it also does zero work. KE is unchanged. Which is correct.
sophiecentaur said:
2. Alternatively, the work done in expelling the ejecta is the force on ship and/or ejecta times distance moved.
The [nearly] instantaneous action of expelling a particular bit of ejecta does the same amount of work in the accelerating frame as it does in the inertial frame. There is no time for acceleration to make a significant difference.
However, a bit of ejecta seen by the accelerating rocket will be accelerating away, gaining kinetic energy as it does so. This is explained by the fictious force from the acceleration of the frame.
sophiecentaur said:
3. Or you could be saying that the work done with the engine is changing the relative velocity of a free object before it hits the floor. But work is still being done on the ship in a conventional way as it accelerates to 'overtake' the free object.
This one.
A falling object is accelerating as viewed by an observer standing on the rocket floor. It is gaining kinetic energy. This is explained by the work done by the fictious force arising from the acceleration of the reference frame.
sophiecentaur said:
How are those three changes in energy not examples of N3?
The first one (object on the floor) did not involve a change in kinetic energy at all. If you think it did, you are frame jumping.
The second one (expelling the ejecta) is an ordinary interaction force. Of course it involves N3.
The second one (watching the ejecta after ejection) does not involve a N3 interaction. The gain in KE is from a fictitious force. Fictitious forces lack a third law partner.
The third one (watching a falling object) does not involve a N3 interaction. The gain in KE is from a fictitious force. Fictitious forces lack a third law partner.
sophiecentaur said:
There is KE 'in abeyance' as the object and ship have relative motion change but that applies to any pair of objects in the Universe that are under different forces. So everything is involved in some sort of work translation with respect to everything else? I see how that's a possible view but is it of any use? You may be able to convince me more. Try.
The only notion of "KE in abeyance" that I know of is potential energy. Some accelerating frames allow a potential to be defined. A uniformly accelerating frame is one example. A rotating frame is another.
In a uniformly accelerating frame, the potential is the familiar ##\text{PE} = mgh##.
In a rotating frame, I believe that the centrifugal potential is ##\text{PE} = -\frac{1}{2}m\omega^2r^2##