To summarize over these four examples one can say that if an object is speeding up to the right, its acceleration is to the right. If an object is traveling to the right but slowing down, its acceleration is to the left. If an object is speeding up to the left, its acceleration is to the left. If an object is traveling to the left but slowing down, its acceleration is to the right. An object in free fall is an object that is under the influence of gravity only.
Air resistance is ignored or is considered negligible.
In this kind of motion, the object is always gaining velocity in the downward direction. From the earlier equation for the definition of average acceleration one gets or since for free fall. For this example the initial velocity is zero. Thus . Consider the previous example with a downward initial velocity of instead of zero. In equation form or .