Math Help (URGENT X10)

This is really urgent so please please please help.

The height(H) of an object that has been dropped or thrown in the air is given by:
H(t)=-4.9t^2+vt+h
t=time in seconds(s)
v=initial velocity in meters per second (m/s)
h=initial height in meters(m)

H=height
h=initial height
Is there a difference but, anyway I didn't make this clear on the last post.

A ball is thrown vertically upwardd from the top of the Leaning Tower of Pisa (height=53m) with an initial velocity of 30m/s. Find the time(s) at which:
a)the ball's height equals the hight of the tower
H(t)=-4.9t^2+30t+53
H(t)=???

b)the ball's height is greater than the height of the tower

c)the ball's height is less than the height of the tower

d)the ball reaches its maximum height

I don't know how to do this problem.


Please Help and Thank You very much =)

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asked by Anonymous
  1. acceleration downward due to gravity is 10m/s to be exact 9.8m/s
    this is enough to get you to start thinking

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    posted by Rami Dahabreh
  2. h(t) = -4.9t^2 + 30t + 53

    a) ball goes up, comes back down to the top of the tower. So, we want

    53 = -4.9t^2 + 30t + 53
    0 = -4.9t^2 + 30t
    0 = t(-4.9t + 30)
    so, t=0 (at the start) or t = 6.12 (as it comes back down)

    If t(-4.9t+30)=0, either
    t=0
    or
    -4.9t+30 = 0
    That is, t = 6.12

    If you can't solve a factored expression, you have some review to do.

    b) same calculation but, t is between 0 and 6.12. That is 0 < t < 6.12

    c) same calculation, but restricting t to positive values, t>6.12
    Naturally, we could also restrict t to the point where height >= 0.

    d) vertex of any parabola is where x = -b/2a = -30/-9.8 = 3.06

    you know from the quadratic formula that x = -b/2a ± sqrt(blah blah)
    Parabolas are symmetric, so the vertex is midway between the roots, which are equally spaced around x = -b/2a

    h(3.06) = 98.9

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    posted by Steve

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