Answer:
(-3/4)x^2 + 6x = 3
Step-by-step explanation:
It takes 8 seconds for the ball to reach its maximum height and then return to ground level; thus, the highest point (vertex) the ball will reach is reached after 4 seconds. The graph showing this motion is a parabola that opens down. Thus, the coefficient 'a' must be negative.
Returning to a * x ^ 2 + bx + c, we write out two instances of this formula, one for x = 0 and the other for x = 8. This is because {0, 8} are the x-intercepts.
Then, for x = 0, a * x ^ 2 + bx + c becomes a * 0 ^ 2 + b*0 = 0,
and for x = 8, a * x ^ 2 + bx + c becomes a(8)^2 + b(8) + c = 0
The first equation tells us that 'c' must be 0. The second equation tells us that 64a + 8b = 0, which is equivalent to 8a = -b, or a = -b/8.
We then have the quadratic function (-b/8)x^2 + bx + 0. All we have to do now is to find the value of b. The vertex of this parabola is (4, 12), and so the quadratic becomes
(-b/8)(4)^2 + b(4) = 12. To solve this for b, multiply all terms by 8:
-b(16) + 32b = 96, or
16b = 96, or b = 6. Since a = -b/8 (see above), a = -6/8 or a = -3/4.
Then the equation of motion is
height of ball = (-3/4)x^2 + 6x
We want to know when the ball is at a height of 3 m. Thus, set this equation of motion = to 3 and solve for x (the times at which height = 3 m):
(-3/4)x^2 + 6x = 3 This is the desired quadratic equation.
