How Do Two Balls Dropped from a Window Hit the Ground Simultaneously?

[mike_302]

Homework Statement

A ball is thrown vertically upward from a window that is 3.6 m above the ground. Its initial speed is 2.8 m/s .

(a)with what speed does the ball hit hte ground?
(b)How long after the first ball is thrown, should a second ball be simply dropped from the same window, so that they both hit the ground at the same time?

Homework Equations

any of the 5 uniform acceleration equations. Gravity is -9.81 (he insists on the 1)

The Attempt at a Solution

I have (a) done... I obtained -8.86 m/s as the velocity when it hits the ground. But for (b), I don't know what method to go about doing this. My gut says there might be something easy, applying the "final speed" that I obtained in (a), but we also learned yesterday to do some graphing methods... Determine when two objects hit the same position, going at this velocity, and the other going at this velocity... That's what I'm trying to do here, but I can't figure it out.

For the dropped ball, the equation would be 0=1/2(-9.81)DeltaT^2 (from d2=v1DeltaT + 1/2aDeltaT^2 + d1 )

But what about the other ball?

 

[Delphi51]

The equation for the other ball would be the same except with a Vi*t term added.
Careful - the T's in the two equations are different. Better use t1 and t2 or something.
Solve each equation so you know t1 and t2. The difference is the answer!