A short Question

Displacement, velocity, acceleration are also positive.
As these three are vectors, when the downward direction is taken as positive, the ball will surely move from top to the ground, because it's travelling direction is downwards, the diplacement will certainly be positive. The velocities either it's initial or final, they will also be positive as well as acceleration.
The following equation shows the relationship among the three vectors:
v= s/t
a= v/t
Where s, v, a and t stand for displacement, velocity, acceleration and time respectively. And the equations show, when s is positive, v will also be positive as well as a.
So, since the downward motion is positive, displacement is therefore positive, initial, final velocity and acceleration are also positive.

2007-10-11 23:32:02 補充：
thz

1c) final velocity: +ve/-ve