I was hoping somebody could help me with a problem I've been working on and

possibly point me in the right direction if I've made a mistake. The

problem reads as follows:

From the top of a cliff, a person uses a slingshot to fire a pebble

straight downward, which is the negative direction. The initial speed of

the pebble is 9.0 m/s (a) What is the acceleration (magnitude and

direction) of the pebble during the downward motion? Is the pebble

decelerating? Explain. (b) After .50 s, how far beneath the cliff top is

the pebble.

For part (a) I have the acceleration of the pebble is 9.8 m/s^2 downward

and that it is not decelerating because the vectors do not point in

opposite directions.

For part (b) my math looks as follows:

x = (v[0])(t) + (1/2)(a)(t^2)

x = (9.0 m/s)(.50s) + (1/2)(9.8 m/s^2)((.50s)^2)

x = 4.5m + 1.2m = 5.7m

I have a fair bit of confidence in my answer for part (b), however it is

part (a) that's been troubling me since I'm not really sure what is meant

by "the negative direction." As far as I understand when gravity causes

ojbects to accelerate then it is +9.8m/s^s and when objects are

decelerating due to gravity then it is -9.8m/s^2. Since the object

accelerates all the way to the bottom of the cliff, I don't see why any

values in this problem should be negative. Can someone please explain this

to me?