When will two balls be at the same height during 1-dimensional motion?

[ksinelli]

Homework Statement

A ball is thrown upward from ground level at 25 m/s. At the same instance a 2nd ball is dropped from rest from a building 15 meters high. How high above the ground will the two balls be when they're at the same place?

Homework Equations

equations of uniformly accelerated motion. don't feel like typing them out. you know what they are.

http://en.wikipedia.org/wiki/Equations_of_motion

The Attempt at a Solution

i've tried listing everything i know about ball 1 and ball 2 and then trying to find some of the things I don't know, but I can't figure out how to do it.

One thing that bothers me is it seems the displacements can never be equal. Since one ball is falling down, its displacement from its original position will be negative. The other ball will have a positive displacement, since it is thrown upward from its original position.

So how would you write what you are looking for mathematically? I initially thought setting the displacements equal to each other and then asking myself "for what value is this equation true?" would work, but as I said, it seems the displacements can never be equal.

I hope I'm making sense. I could really use some help.

 

[weesiang_loke]

hi,
for me i will set the displacement for both ball with the ground as the reference point.
so for the 1st ball we have the equation:
displacement at time t, s1 = 25*t - 0.5*g*(t^2)

and for 2nd ball:
displacement at time t, s2 = 15 - distance traveled in time t
s2 = 15 - o.5(g)*(t^2)

so when the balls meet, the displacements are equal. then we will have the time when that happens.
thus we can find the height from the ground.

hope that help.