# Inertial reference frames

Hello, I am having difficulty understanding the concept of Newton's first law only applying in an inertial reference frame, or a frame that is at constant velocity, however, apparently the 1st law no longer applies if the reference frame is accelerating. Can anyone give me some sort of concrete example so I can see it in my mind, right now it's not making any sense to me. Actually, I just need an explanation of the whole concept of inertial vs noninertial reference frame.

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