# Find the buildings height for two balls to land @ same time

1. Sep 2, 2015

### proto3210

1. The problem statement, all variables and given/known data
A ball is thrown straight up from the edge of the roof of a building. A second ball is dropped from the roof 1.00s later.

A) If the height of the building is 20.0m, what must the initial speed of the first ball be if both are to hit the ground at the same time?

B) Consider the same situation, but now let the initial speed v0 of the first ball be given and treat the height h of the building as an unknown. What must the height of the building be for both balls to reach the ground at the same time if v0 is 6.0 m/s.

2. Relevant equations
Δx = v0t + ½at2

3. The attempt at a solution

For part A, I used this equation. Δx = v0t + ½at2 and set x= -20m and found out that the initial velocity needed for ball 1 in order for both balls to land at the same time would be 8.11 m/s.

For part B I am lost, I'm not sure where to begin.

Last edited: Sep 2, 2015
2. Sep 2, 2015

### Orodruin

Staff Emeritus
There is nothing different in B. The only different thing is which variable to solve for.