Free fall is what happens when an object is let to fall on its own without propulsion or resistance. Someone who is standing is not in free fall, but someone jumping from an aeroplane is, until air resistance builds up, ending in terminal velocity. An artillery shell is in free fall from the time it is fired until it hits something. Even the time when it is going up is free fall. Space stations and spacecraft are in free fall when their engine is not on and they are not sitting somewhere, which means all or most of the time. For Earth the acceleration due to gravity (g) is 9.81 Newton (unit) per kilogram.