Ok I'm really trying to understand inertial and non-inertial reference frames, my understanding is as follows: A rest observer on the earth, the observer will be stationary relative to the earth.. Now as I understand it an inertial reference frame is one of which 2 coordinate systems are both inertial relative to eachother? This means that the observer would be inertial right? Because the observer is stationary (inertial) relative to the earth and visa versa? I wasn't sure how to take the earths rotation into account because i'm not talking about the observer relative the sun or any other celestial object, therefore confined to the reference frame of the observer and earth, any rotation is completely unaccounted for right? As if the earth wasn't rotating? Now another example is an observer on a merry-go-round. Would this be the same principle or not? Because the observer would be stationary relative to the merry go round and visa versa once again? So the observer would be inertial? Now for a non-inertial example, if the observer is on the ground at rest next to the merry go round, the observer is now accelerating relative to the merry go round and the merry-go-round accelerating relative to the observer right? So the observer is now in a non-inertial reference frame? Have I got this correct or is something misinterpreted? It just seems too obvious to me but I know these frames of reference can be quite complicated? Confused.