 We first saw that when you describe a system in terms of coordinates, the actual coordinates of an object don't really mean anything physically because they change depending on which reference frame you're in. And there's no real reason to fundamentally prefer any one reference frame over another other than because it's convenient to the problem at hand. What matters are distances between objects because these are the same regardless of reference frame. We then saw that a similar thing applies to velocities. The only thing that's really physically significant are relative velocities because the actual velocity changes depending on which frame you're in. To make a meaningful statement, you need to say something like Alice's stationary relative to the train or Alice's stationary relative to the tracks. Now these ideas, the main things that are important are relative positions and relative velocities form the basis of what's called Galilean relativity. Now an important thing to note is that all the reference frames we've been considering so far have involved one travelling relative to the other at a constant velocity, so there's been no acceleration and no turning, which is really just another form of acceleration. Now this is important and these kinds of reference frames are called inertial reference frames. So you have two reference frames moving relative to the other at constant velocity. Now as we study special relativity we'll restrict ourselves for the most part to inertial reference frames. Trying to deal with acceleration is quite difficult and it took Einstein 10 years to figure out how to do that and the result is what we call general relativity which we might talk about towards the end of the course.