Let’s assume we have 2 different observers. Observer 1 sits in space and observer 2 sits in a space lab which is in a free fall state toward the Earth. We further assume that observer 2 in the space lab does not have any information regarding its surrounding. (e.g Space lab has no windows and observer 2 does not know that there is planet around him which causes any gravitational force).
- Is there anyway that observer 2 can figure out whether any frame attached to the space lab can be regarded as an inertial or non-inertial frame? (According to the equivalence principle this should not be possible. Is this true?)
- Lets assume there is a 1kg ball inside the space lab which is also free falling together with the space lab? Observer 1 (with its true inertial frame) can measure the acceleration of the ball and deduce that 9.8Newton force is acting on the ball. On the other hand, observer 2 measures the acceleration of the ball as 0, and hence, deduces that the net force acting on the ball is 0. As the observer 2 has no way of knowing that space lab is not an inertial frame, he will not have any doubt about his force measurement. Does this mean that definition of force is dependent on the frame of reference?
Update after lesnik's answer:
- Is it possible to define an experiment inside the space lab which proves the observer 2 that he is in fact accelerating? From my understanding, as long as it is assumed that gravity field is uniform, all 3 laws of Newton hold in this free falling space lab without requiring any fictitious force definition. In this case, why cannot we clasify this free falling body as an inertial frame of reference in Newtonian Context?