Acceleration of water drops problem

[abhikesbhat]

Homework Statement

If the effects of air acting on falling raindrops are ignored, then we can treat raindrops as freely falling objects. Rain clouds are typically a few hundred feet up in the sky. Estimate the speed which raindrops would strike the ground if they were freely falling objects.

Homework Equations

vf^2=vo^2+2as
vf-TARGET
v0- 0
s- 200
a- -9.8

The Attempt at a Solution

Ok I know the answer, but I don't understand something. What I did was put all my knowns into the equation. Then this is what it looks like.
vf^2= -3920
That is obviously unreal. Then I see that there is a problem with the acceleration. WHY? I thought acceleration of freely falling objects was -9.8.

 

[abhikesbhat]

Oh and by the way this is not homework. My goal is to make the iPHO in my high school years. This problem is from University Physics. Which book do you pros recommend to prepare for olympiad problems? University Physics or Fundamental Physics or others?

 

[Redbelly98]

s and a are both negative here.

 

[klite]

If I'm not mistaken, the acceleration of freely falling objects near the surface of the Earth is 9.8 m/s2, so a would be 9.8, try plugging that into the equation.