The acceleration of gravity is 9.8m/s² downward.
That means that if no other vertical forces but gravity act on an object,
then the object's downward speed increases by 9.8m/s every second,
or its upward speed decreases by the same amount every second.
So if a rock starts out moving in the upward direction at 11.5 m/s, and the
only vertical force on it is the force of gravity, then it will lose 9.8 more m/s
of upward speed every second, and will totally run out of upward gas after
11.5 / 9.8 = 1.173 seconds .