# Ball Drop Problem

1. Sep 9, 2008

### ArbazAlam

A ball is thrown straight up from the edge of the roof of a building. A second ball is dropped from the roof 2.00 s later. Air resistance may be ignored.

(a) If the height of the building is 60 m, what must be the initial speed of the first ball if both are to hit the ground at the same time?

I've done a number of things that have given me very strange(improbable) numbers. Now what I've attempted to do is set up an equation for each of the balls.

y1(t) = -4.9(t+2)2
y2(t) = vot - 4.9t2

I'm not sure where to go from here. I tried setting y1 to 60 and solving to find the time and then plugging it into y2 but that turned out to be incorrect.

2. Sep 9, 2008

### PhanthomJay

You've got your times reversed and you must watch your plus and minus signs. The second ball is thrown later. If it takes the second ball , which you are calling 1, 't' seconds to reach the ground, the first ball has t + 2 seconds to reach the ground. Also, what is the direction of the displacement?