Velocity?

Kimmysmiles0 · April 27th, 2012, 09:09 PM

I think this is needing velocity found before anything can be done but I am completely lost. Any help would be greatly appreciated.

Set up an equation and then solve this problem:

An object is dropped from the top of a 400 foot-tall building. The height h (in feet) of the object t seconds after it is dropped is given by
h(t)= -16t^2 + 400

How long does it take for the object to hit the ground?

MarkFL · April 27th, 2012, 09:14 PM

You don't need the velocity, just set $h(t)=0$ and solve for t. You will only need the positive root.

$h(t)=0$

$400-16t^2=0$

Divide through by 16 and arrange with the variable on the left:

$t^2=25$

Take the positive square root:

$t=5$

Hence, the object will hit the ground 5 seconds after being dropped.

April 27th, 2012, 09:09 PM	#1
Kimmysmiles0 Newbie Joined: Apr 2012 Posts: 14 Thanks: 0	Velocity? I think this is needing velocity found before anything can be done but I am completely lost. Any help would be greatly appreciated. Set up an equation and then solve this problem: An object is dropped from the top of a 400 foot-tall building. The height h (in feet) of the object t seconds after it is dropped is given by h(t)= -16t^2 + 400 How long does it take for the object to hit the ground?
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April 27th, 2012, 09:14 PM	#2
MarkFL Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 462 Math Focus: Calculus/ODEs	Re: Velocity? You don't need the velocity, just set $h(t)=0$ and solve for t. You will only need the positive root. $h(t)=0$ $400-16t^2=0$ Divide through by 16 and arrange with the variable on the left: $t^2=25$ Take the positive square root: $t=5$ Hence, the object will hit the ground 5 seconds after being dropped.
$MarkFL is offline$

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