kindly keep the explanation rather simple plz
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For an ELI5, its when an observer can not feel a force.
Floating in space you are in an inertial frame.
Moving at constant speed in space you are in an inertial frame. Indeed these first two cases are indistinguishable if there is nothing for the observer to see.
Standing on earth we are NoT in an inertial frame. You can feel the force of gravity on your feet.
Falling in a gravitational field you are in an inertial frame (ie being in orbit around a planet).
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