Position when 2 objects pass each other

[dvdqnoc]

Homework Statement

An object is thrown downward with an initial speed of 4 m/s from a height of 99m above the ground. At the same instant, a second object is propelled vertically from the ground level with a speed of 25 m/s.
The acceleration of gravity is 9.8 m/s^2.
At what height above the ground will the two objects pass each other? Answer in units of m.

Homework Equations

http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L6a1.gif

The Attempt at a Solution

Well I tried to find a common variable to plug in. I tried to plug in the position, then find time, then find the position, but it didnt work. Any help is appreciated.

 

[learningphysics]

Can you show exactly what you did?

 

[dvdqnoc]

What I did didn't work. I kind just mushed stuff together. I might as well have said "I didnt know where to start" because I didnt.

 

[learningphysics]

What I did didn't work. I kind just mushed stuff together. I might as well have said "I didnt know where to start" because I didnt.

the equation to use is:

d = v1*t + (1/2)at^2

now use d = X - Xo

X - Xo = v1*t + (1/2)at^2

so: X = Xo + v1*t + (1/2)at^2

X is the height at any time t. Xo is the initial height.

use this to get the displacement equations for each of the 2 objects...

after you do that, set them equal... solve for t...