AP® Calculus AB-BC

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Firework Launch Into a Lake


Jacob launches a firework from the edge of a cliff overlooking a lake. It does not take long for him to realize that the firework is a total dud, because rather than exploding in a gorgeous display of light and color, it plummets into the lake without a flicker.

The height of the firework, measured in feet, is given in the table below, where $t$ is the number of seconds after the firework is launched.

$$ t \hspace{1.5cm}$$ 0 1 2 3 4 5 6 7 8 9
$S(t)$ 144 256 336 384 400 384 336 256 144 0

At what time does the vertical velocity of the defunct firework equal zero?


$t = 0$


$t = 4$


$t = 8$


$t = 9$