# Free fall question

1. Oct 25, 2008

### msc8127

1. The problem statement, all variables and given/known data
a ball is thrown straight down from the top of a 220 feet building with an initital velocity of -22ft/s. What is the velocity of the ball after 3 seconds? What is the velocity of the ball after falling 108 feet?

2. Relevant equations
s(t) = -16t^2 + Vi(t) + si vi = initial velocity, si = initial position

3. The attempt at a solution
s(t) = -16t^2 +Vi(t) +si
= -16t^2 - 22ft/s + 200ft

V(t) = -32(t) - 22ft/s
V(3) = -96 ft/s - 22ft/s = -118 ft/s

I can seem to figure out how to determine the velocity of the ball after it has dropped 108ft.

I should be able to do this, but i'm missing something, somewhere on what should be pretty basic.

Thanks for any guidance!

2. Oct 25, 2008

### nrqed

There are two ways to do it.

The quickest way is to use $$V_f^2 = V_i^2 - 2 g \Delta y$$.

The other ways is to use the equation $$y= V_i t -1/2 g t^2$$ to first find the time to drop 108 feet and then to plug this into $$V_f = V_i - g t$$

3. Oct 25, 2008

### msc8127

Thanks for the reply. I have a solution that makes sense now. :)