In the formula vy=a t what is the value of a for an object in projectile motion

I THINK that formula gives the vertical speed of a projectile at the time
't' seconds after it's launched. 

(I can't really be sure of anything, because whenever you show a formula
to other people, you're supposed to tell them what each variable in it means,
and you didn't do that.)

If I'm on the right track, then the 'a' is the acceleration of gravity wherever
the projectile is flying.  If that happens to be on Earth, then the acceleration
is  9.8 meters per second² .

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.