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Math Help - projectile

  1. #1
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    projectile

    a projectile is shot upwards with an initial velocity of 30m/s , its height at time t is given by h=30t-4.9t^2, during what period of time is the projectile more then 40m above the ground? express your answer rounded to the nearest hundredths

    my thinking: 40=30t-4.9t^2 but can't go anywhere from there
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  2. #2
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    Quote Originally Posted by william View Post
    a projectile is shot upwards with an initial velocity of 30m/s , its height at time t is given by h=30t-4.9t^2, during what period of time is the projectile more then 40m above the ground? express your answer rounded to the nearest hundredths

    my thinking: 40=30t-4.9t^2 but can't go anywhere from there
    So  4.9t^2-30t+40=0
    Use the quadratic formula: t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}
    a=4.9 b=-30 c=40
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  3. #3
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    Quote Originally Posted by JeWiSh View Post
    So  4.9t^2-30t+40=0
    Use the quadratic formula: t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}
    a=4.9 b=-30 c=40
    so it was above 40m between 1.9 and 4.1 seconds?
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  4. #4
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    Quote Originally Posted by william View Post
    so it was above 40m between 1.9 and 4.1 seconds?
    Correct
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