Right clicking a keyframe on pan-crop you have options for linear, fast, smooth, sharp, and hold. which of these would be best for simulating an object falling..i.e. gravity? and how to use on start and end key frames? and how to use in combo with the keyframe interpolation smoothness?
linear, fast, smooth, sharp, and hold. - Not sure for gravity? In as much as the need for getting the "control" you might want.
However I WOULD use the "Linear" to initially to obtain a linear/standard motion set-up, meaning going from A > B.
Then, as a side step approach to this, I would employ the velocity envelope, applying closer=faster change points on a hyperbolic shaped curve gradient. And NO we do not have these as presets, or such like. Manual manipulation here. This will produce/simulate the force of gravitational acceleration - an ever increasing increasing velocity(acceleration).
While Grazie has it nailed exactly on the head and told you the right way to do it ... ;)
If you do want to stick with only keyframe settings and not use velocity envelope then start with smooth and end with fast. You won't like the result though so go back and read what Grazie said again.
Thanks guys... I'm afraid I'm going to have to stick with keyframes as I have way to many moving events that need to stay together and velocity envelopes are a bear to control.