# Homework Help: Free Fall

1. Sep 28, 2008

### Thiendrah

1. The problem statement, all variables and given/known data
A ball is thrown vertically upward with an initial velocity at the edge of the roof of a building which is 30m above the ground. The speed of the ball is 14.5 m/s when the ball is 12m above the ground. How long does the ball take to reach maximum height?

2. Relevant equations

3. The attempt at a solution
First, i use that equation to find distance between the highest point and 12 above ground. Then I plan to use it one more time to find the initial velocity. But the thing is that

I find d=-10.727m as the distance between highest point and 12 above ground. This doesn't make sense, shouldn't it be higher than 18m?

2. Sep 29, 2008

### tiny-tim

Hi Thiendrah!

Show us your calculations, and then we'll know how to help.

3. Sep 29, 2008

### minifhncc

v2=u2+2aS

Use that equation for all the calculation 12m above the ground and then add 12m to your final result.

So obviously v=0 (as it reaches the top) u=14.5ms-1 a= -9.8ms-1 (If up is positive), and then solve for S and add 12m.

4. Sep 29, 2008

### Thiendrah

minifhncc, that's the same thing. You still get the same result.

0^2=14.5^2+2.(-9.8).S

S=10.727

10.727 + 12 = 22.727...

this doesn't make sense tho. The object is thrown vertically upward off a roof that's 30meters above the ground.

Am i doing anything wrong?

5. Sep 30, 2008

### tiny-tim

Hi Thiendrah!

No … you seem to be right.

I can't see any way out of this in the question …

the question specifically says that the initial velocity is vertical …

your formula is correct (you could also have used KE + PE = constant, with the same result) …

the question is wrong.