I am not so clear about the definition of 'free fall'. If an object is an example of free fall, then must its acceleration be 9.81m/s^2? For example, if a sky diver jumps from an airplane, because earth has atmosphere, the rate of acceleration decreases due to air resistanc. so now the acceleration is not equal to the gravity that the earth pulls it down, then is it still a free fall? Second example, a parachutist descending at a speed of 10.0m/s loses a shoe at an altitude of 50.0 m. My teacher said it was not a free fall because it didn't start at rest. Must a free fall motion start at rest? but the acceleration is still 9.8m/s^2. so even when it is not a free fall, its acceleration can still be equal to gravity that pulls it downward? thanks a lot.