• Dec 29th 2010, 09:29 PM
damocles
Hi, i'm very new to this forum and i am stuck on this problem. Perhaps i'm just having a brain freeze, but i'd like to know how to solve this question.Thanks :). And apologies if this is in the wrong part of the forum.

The height, h(t) metres, of a batted baseball as a function of the time, t seconds, since the ball was hit can be modelled by the function:

$h(t)=-2.1(t-2.4)^2 +13$

Question: How many seconds after it was hit did the ball hit the groud, to the nearest tenth of a second?

The answer key tells me 4.9 seconds, however, i am unable to arrive at that answer. My idea was to substitute h(t) with 0 to get the x intercept, but the numbers greatly confused me.

Any help? Thanks in advance :)
• Dec 29th 2010, 10:16 PM
dwsmith
h(t) = 0 the ball hits the ground.

$\displaystyle 0=-2.1(t-2.4)^2+13\Rightarrow \frac{13}{2.1}=(t-2.4)^2\Rightarrow\sqrt{\frac{13}{2.1}}+2.4=t$

$\approx 4.888066$
• Dec 29th 2010, 10:17 PM
damocles
gah. solved lol. thanks xD. i missed the square root step xP.