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 airplane 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. Spacecraft are in free fall when their engine is not on and they are not sitting somewhere, which means most of the time. In most mathematical calculations the acceleration due to gravity (g) is 9.81 Newton (unit) per kilogram.