In nature, it is very rare to see an object moving in a constant speed because every motion is impacted by gravity, air resistance, and friction.
This is why UI transition in a constant speed appears weightless.
"Forth down at a half time" is a rule in expressing the animation of a falling object.
In many prototyping tools such as Adobe Edge Animate and Axure, the canned animation effects support this already, with the terminology of "ease in" and "ease out".