Question

What are you understand by a frame of reference, and what are inertial frames?

Answer 1

A frame of reference is where you observe something and make some measurement.

A inertial frame is a frame of reference where the laws of physics apply in it's simplest form, like F=ma. It is a frame that has no acceleration.

If you are making an observation on a train on smooth tracks that moves with constant velocity, you are in an inertial frame. If you did experiments you would find that F=ma.

However, most tracks are not smooth and you run into bumps. This bumps cases acceleration to you. But if you are in the train, your frame of reference now is no inertial, and thus you swing back and forth due to the bumps, but you cannot explain where this force comes from. And so a term ficticious force comes about to explain why you are thrown around.

But if you are outside the train standing on Earth, you are still in an inertial frame (approximately) and so you can observe that the person in the train is not bumped around due to forces: he is bumped around because he still moves with his original velocity.

Answer 2

thanks Festinger.