# Help with parabola

1. Jan 12, 2009

### Blueberry91

1. The problem statement, all variables and given/known data

In an over hand serve, the ball starts from a point 2m above the ground and travels a horizontal distance of 20m before it strikes the floor.

The ball travels in a parbolic path and reaches its highest point after travelling a horizontal distance of 9m as show in the link below:

http://img407.imageshack.us/my.php?image=asdaq2.jpg

Determine the equation of the parabola in the form y-k=a(x-9)^2

2. Relevant equations

y-k=a(x-h)^2

3. The attempt at a solution

Well i know that the ball reaches its maximum height 9 feet from
where it was served. If we let x=0 be where the ball was served,
then we know the x coordinate of the vertex is 9, so the equation is
of the form

y-k=a(x-9)^2

To finish the equation precisely, I need to determine the values of
a and k. To do that, I need two equations relating the values of a
and k. But the given information provides with what I need to
write those two equations --I know the x and y values (1) where
the ball was served and (2) where it hit the ground.

but i'm still confused ! i could use some help:)
could somebody solve it for mee pleaaase??

2. Jan 12, 2009

### Blueberry91

Re: Parabola?

btw i missed the points on the Y axes
there's 0,10 and 20

the value 10 is aligned to the point V
does that mean that (9,k) V is equal to (9,10)
and so with the given value of k i just solve for the a value?
if so, i get a very awkward answer:S

3. Jan 12, 2009

### Dick

Re: Parabola?

Ok, so y-k=a(x-9)^2. You know x=0, y=2 and x=20, y=0 satisfy that equation. Doesn't that give you two equations in two unknowns k and a to solve? What are they?

4. Jan 13, 2009

### Staff: Mentor

Re: Parabola?

That would be 9 meters.