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Thread: integration problem (pls help)

  1. #1
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    integration problem (pls help)

    If a ball is thrown vertically upwards from a platform 8 feet above the ground. After 2 seconds, the ball is 2feet below the platform. Find the initial velocity of the ball. (Acceleration due to gravity = -10m/s^2)

    Answer key: 31m/s

    how????
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  2. #2
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    Re: integration problem (pls help)

    $s(t) = s_0 + v_0 t + \dfrac 1 2 g t^2$

    $a \approx -10m/s^2$

    $s_0 = 8ft$

    $s(2) = 8-2=6ft$

    Just plug everything in and you'll have some simple algebra to find $v_0$
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  3. #3
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    Re: integration problem (pls help)

    In still confused cause I keep ending up with 9m/s

    6 = -5(2^2) +2Vo +8
    9 = Vo
    Last edited by Maqi; Apr 10th 2019 at 03:14 PM.
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  4. #4
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    Re: integration problem (pls help)

    $v_0 = 9 \, m/s$ is the correct solution for the given parameters. If the initial velocity were 31 m/s, the projectile would take slightly more than 3 seconds to reach the top of its trajectory.
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  5. #5
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    Re: integration problem (pls help)

    Quote Originally Posted by Maqi View Post
    In still confused cause I keep ending up with 9m/s

    6 = -5(2^2) +2Vo +8
    9 = Vo
    oh... it looks like there's some units conversion to be done.

    They give the initial and final positions in feet but the acceleration due to gravity in m/s.

    Convert the positions to meters.
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  6. #6
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    Re: integration problem (pls help)

    Ohhhh okay thanks so much. I finally got it!
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