My Math Forum Velocity?

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 April 27th, 2012, 10:09 PM #1 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?
 April 27th, 2012, 10:14 PM #2 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 463 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.

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