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Math Help - [Kinematics] Total time in air

  1. #1
    Junior Member Cthul's Avatar
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    [Kinematics] Total time in air

    Given that a ball is thrown vertically up.
    The ball reaches 78.4\text{m} as a maximum height. (Gravity is 9.8)

    Find the total time the ball was in the air.
    The information given is:
    <br />
a=9.8\text{m}\text{s}^{\text{-}2}<br />
    <br />
s=78.4\text{m}<br />
    <br />
u=39.2\text{m}\text{s}^{\text{-}1} (I worked out the initial speed, it's correct)
    <br />
v=0{m}\text{s}^{\text{-}1}<br />
    <br />
t=x<br />
    For the ball going up only.

    How do I find the rest of the time?
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  2. #2
    Flow Master
    mr fantastic's Avatar
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    Quote Originally Posted by Cthul View Post
    Given that a ball is thrown vertically up.
    The ball reaches 78.4\text{m} as a maximum height. (Gravity is 9.8)

    Find the total time the ball was in the air.
    The information given is:
    <br />
a=9.8\text{m}\text{s}^{\text{-}2}<br />
Mr F says: If you want s = 78.4, then a = -9.8, not 9.8. s and a are vectors and in straight line motion direction is specified by + or -.

    <br />
s=78.4\text{m}<br />
    <br />
u=39.2\text{m}\text{s}^{\text{-}1} (I worked out the initial speed, it's correct)
    <br />
v=0{m}\text{s}^{\text{-}1}<br />
    <br />
t=x<br />
    For the ball going up only.

    How do I find the rest of the time?
    In the absence of air resistance, time up = time down.
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  3. #3
    Junior Member Cthul's Avatar
    Joined
    Mar 2010
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    I can't believe that was so easy. Thanks.
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