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Math Help - word problem help

  1. #1
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    word problem help

    Hi, can someone help me with this? I don't even know how to set up the equation.

    If an object is propelled upward from a height of 96 feet at an initial velocity of 80 feet per second, then its height h after t seconds is given by the equation h = - 16t2 + 80t + 96. After how many seconds does the object hit the ground?
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  2. #2
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    You already have the necessary equation: h(t)=-16t^2+80t+96. Hitting the ground means the height at which the object is is 0: h(t_0)=0 where t_0 is the time it hits the ground (assuming it gets launched at t=0).
    Last edited by Nyrox; February 12th 2010 at 04:32 AM. Reason: Typo
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  3. #3
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    Quote Originally Posted by EandH View Post
    Hi, can someone help me with this? I don't even know how to set up the equation.

    If an object is propelled upward from a height of 96 feet at an initial velocity of 80 feet per second, then its height h after t seconds is given by the equation h = - 16t2 + 80t + 96. After how many seconds does the object hit the ground?
    h=-16t^2+80t+96

    as this is the height equation,
    then when the ball first hits the ground, the height will be zero.
    If you write the equation as a multiplication (factorise), the answer will be in one of the factors.

    h=0\ \Rightarrow\ -4t^2+20t+24=0\ \Rightarrow\ t^2-5t-6=0

    (t-6)(t+1)=0

    This is zero for t=6 seconds and t=-1 second.
    The t=-1 comes from the graph which can show times "preceding" when the ball was propelled!!

    Hence the answer is 6 seconds.
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  4. #4
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    thx

    Thanks for your help!
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