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* .