# Homework Help: Freely falling bodies

1. Oct 15, 2008

### star20

1. The problem statement, all variables and given/known data

A ball is thrown from top of a 25m tall building. The balls initail speed is 12m/s. At the same instant a person is running on the ground at the distance of 31m from the building. What must be the avg speed of the person if he is to catch the ball at the bottem of the building?

2. Relevant equations
Ball
V initial = +12m/s
x= 25m
t=?
a= -9.8m/s^2

so i use the equation V^2=Vinti^2+ax to find the final v
and then use the Vfinal= v initial+at to find the time the ball hits the ground
and use this time to find the avg velocity of the boy running to the building

3. The attempt at a solution
Am i taking the right steps thank you

2. Oct 15, 2008

### mgb_phys

Do you need the final velocity?
You know Vinit and distance - you want time
What's the motion equation for distance/time/Vinit ?

ps V^2=Vinti^2+2ax by the way.

3. Oct 15, 2008

### star20

do i use the equa y=vinitial t +.5 at^2

4. Oct 15, 2008

### mgb_phys

Yes, watch the signs of v and a