If we assume that the acceleration is constant the equation of motions are:Originally Posted byQuick

The first thing to do is set up a coordinate system. I will choose upward as the positive direction and the ground as the origin.

So, we know that the original position y0 is at 100 m. The initial speed is (presumably) 0 m/s because we are dropping, not throwing, the object. The acceleration is (note) -10 m/s^2 since the acceleration is in the downward direction. We want t when y = 0.

The obvious equation to use, then, is the first equation.

or s. We obviously discard the negative solution.

-Dan