Projectile motion: Two Balls and Building

[yaniv arazi]

Homework Statement

1.[/B]
two balls are thrown vertically from two points on a building. at the bottom of the building ball 1 is thrown and from the roof ball 2 is thrown. ball one reaches its maximum height after three seconds and ball two after one second. after two seconds the balls cross paths. find the height of the building, the height of the meeting point, when they land and how long they are in the air for. if i am wrong please explain why. thank you in advance.air resistance is neglected.
2.
how do i prove the relationship

where h is the height v is velocity g is gravity and x is distance

Homework Equations

The Attempt at a Solution

height of the building=40 meters
ball one hits the ground after 6 seconds
ball two hits the ground after 4 seconds.
max height of ball one=85 meters
max height of ball two=45 meters[/B]

 

[PeroK]

All but one of the answers look right.

 

[yaniv arazi]

All but one of the answers look right.

could tell me which one and why.thank you

 

[PeroK]

could tell me which one and why.thank you

There one that is obviously incompatible with the others.

 

[yaniv arazi]

There one that is obviously incompatible with the others.

oh yes height of ball one thanks. do you understand question two because i do not

 

[PeroK]

oh yes height of ball one thanks. do you understand question two because i do not

I don't know what question 2 means.

 

[gneill]

Moderator's note: Thread title changed to make it more descriptive of the problem posed.

 

[yaniv arazi]

I don't know what question 2 means.

Question 2 is the question in which i ask how i prove the relationship its marked under question two. Again, thanks for the help sir!

 

[PeroK]

Question 2 is the question in which i ask how i prove the relationship its marked under question two. Again, thanks for the help sir!

Yes, I know, but I don't recognise that equation. Where did you find it?

 

[jbriggs444]

how do i prove the relationship
$$x^2=\frac{2\ \cdot\ v^2}{g}\ \cdot\ h$$
where h is the height v is velocity g is gravity and x is distance

x is the distance from where to where? v is the velocity of what? h is the height of what?

Edit: I think I can derive this formula under the assumption that x is the total distance traveled by a ball thrown vertically upward and then caught again at the point of the throw.

Hint: $x=2h$