There are many important examples where an object experiences constant acceleration, most notably an object in ``free fall''. That is, and object that has been thrown into the air and is not traveling so fast that air resistance becomes important.
In the case of free fall on the surface of the earth, the
acceleration is approximately 
, or 
.
This is for objects moving vertically up or down, so our
coordinate x is now in the vertical direction. (Actually
we'll tend to use y to denote motion in this direction.
Note that one can use any symbol that one pleases such as 
but that would be silly).
We'll study the case of motion with constant acceleration in some detail, first in one dimension, and then in two and three dimensions.