Amazing course! Quick question regarding inertial vs non-inertial reference systems. Because you always measure your velocity respect a reference system, the concept of "constant velocity" depends on what reference system you are using, aren't you?. If the reference system is accelerating then your velocity wouldn't be constant, so you can't be considered an inertial frame either respect to this one. What I am confused with is that even the concept of inertial / non-inertial frame seem to be relative too.

Amazing course!! Quick question regarding inertial and non-inertial reference systems. Say a system is inertial means that it is at constant velocity, right? But, constant with respect to what? Wouldn't be the case that be an inertial / non-inertial is also relative? So you can be inertial respect to some system and non-inertial respect a different one?, I.e. the concept of inertial non-inertial is also relative?

In this discussion, the reference frame is what is inertial not you. Inertial in this context means that the reference frame is moving at a fixed speed in a fixed direction with respect to (all) other inertial reference frames. If you are moving at a constant speed and direction with respect to to some inertial reference frame R then you are moving at some constant speed and direction with respect to any other inertial reference frame, in particular the inertial reference frame R' which is moving with the same constant speed and direction with respect to R. In R' you are stationary. No reference frame has primacy so you can take any reference frame to define your "true" speed including the reference frame where you are stationary. Note that you can take either inertial reference frames or acceleration as the basic concept and define one in terms of the other: (1) an inertial reference frame is one which is not accelerating, or (2) acceleration is what happens when a reference frame changes direction and/or speed, You are quite correct in inferring that the movement of inertial reference frames is a relative concept. Movement is defined relative to that of other reference frames.

@Varunkumar Kothapally We know that e = mc^2 Also, we know that e = 1/2mv^2 So if velocity increases, energy increases. And the speed of light is a constant. So, m = e/c^2. Thus when velocity increases, energy increases and so does mass. I hope that clears the question

So, if you only have two frames of reference, both appear to accelerate with respect to one another, you can never tell which one is inertial?!!!