First, let us be clear by what we mean by "free fall". Free fall is when an object accelerates downwards from rest due to gravity and gravity alone. For this idealized case of free fall, we shall ignore air resistance and assume that the acceleration due to Earth's gravity is constant. This allows us to describe free fall as an application of kinematics, where the initial velocity is 0 and the acceleration is g (roughly 9.8 m/s^2). "g" may be taken to be positive or negative depending on exactly how you've defined your directions.