Velocity is the rate of change of position, and acceleration is the rate of change of velocity.
From these rules we see that for any given situation, the signs will depend on the choice of coordinate axis. But our description of what actually happens must be the independent of that choice. Denote vertical position coordinate y, velocity v y and acceleration a y . These are the general expressions obeyed by any object moving with constant acceleration. In order to complete them for the problem at hand we must choose the origin y 0 of our coordinate axis and the direction in which y increases along that axis.