When does the potential energy become negative?

There is no such thing as absolute potential energy. Potential energy only makes sense as a change in energy. read more

Potential energy only makes sense as a change in energy. If potential energy is decreasing (for instance, if a ball is rolling down a hill), then the change in potential energy will be negative. You can, if you wish, define a reference point to be 0 and then any potential energy below that can be considered negative with respect to that point. read more

At the bottom of the building the potential energy is 0 while the kinetic energy is (1/2)v 2 and so the total energy is (1/2)v 2. Neglecting air resistance, by"conservation of energy" we have (1/2)mv 2 = 980 so v 2 = 1960 and v= 44.2 m/s. read more