Discrepancies with velocities and displacement

Question

 From a 100 m apartment balcony, you throw a tennis ball down at 5 m/s. How long until it hits the ground below?

 

I'm confused on how to assign their directions

Answer 1

s(t) = 100 - ½gt² - 5t   when t-0, s(0) = 100.  This will decrease with time. Sign of velocity term (5t) is negative because time will cause height to decrease. The gravity term, where g = 9.81 m/sec, is also negative because gravity will pull downward also.

 

Solve the equation for t when s(t) = 0.  USE WHATEVER g VALUE IS TYPICAL IN YOUR CLASS; some books use 9.8, others may use g=10 for the sake of convenience.