Posted by jose on .
you are standing on a 100m tall buiding above a street and your friend is standing vertically below the building. When you drop a stone, your friend simultaneously throws the identical stone as yours at a speed of 50m/s in the same straight line. Calculate the time taken for the stones to collide

applied physics 
drwls,
Let the heights of the two stones above the ground be Y1 and Y2.
Y1 = 100  4.9 t^2 (the dropped stone)
Y2 = 4.9 t^2 + 50 t (the thrownupward stone)
Now set Y1 = Y2 and solve for t.
The t^2 terms drop out and you get an easy linear equation to solve.
Enjoy!