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Help with this Physics Problem!!!! Help!!?

Please help with this problem AND SHOW YOUR WORK PLEASE!

A ball is thrown upward from the ground with an initial velocity of 25.0 m/s; at the same instant, a ball is dropped from a building 15 m high. At what time will the balls be at the same height?

Update:

Thanks for the answers,

btw, here's a link to my other physics question - http://answers.yahoo.com/question/index;_ylt=AtVf....

all help is appreciated

2 Answers

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  • 8 years ago
    Favorite Answer

    The general equation for height of the balls is:

    h = Ho + u×t + a×t²/2

    where

    h = height

    Ho = initial height

    u = initial velocity

    t = time

    a = acceleration by gravity = -9.8 m/s²

    The equation for displacement of the upward going ball is:

    h = 0 + 25×t + (-9.8)×t²/2

    h = 25t - 4.9t²

    The equation for displacement of the dropped ball is:

    h = 15 + 0×t + (-9.8)×t²/2

    h = 15 - 4.9t²

    We want the height of both balls to be the same, so

    25t - 4.9t² = 15 - 4.9t²

    25t = 15

    t = 0.6 s < - - - - - - - - - - - - answer

  • 8 years ago

    Same place <--- Equation you need for each ball

    At what time <--- Variable you need to solve for

    Equation of motion

    s = si + vi*t + 1/2*a*t^2

    For this problem

    h = hi + vi*t - 1/2*g*t^2

    Equation for ball being dropped, vi = 0

    1) h = hi - 1/2*g*t^2

    h = 15 - 4.9 * t^2

    Equation for the ball being thrown up... hi = 0

    2) h = vi*t - 1/2*g*t^2

    h = 25.0 * t - 4.9 * t^2

    Since they meet at the same height, set the equations equal and solve for t

    15 - 4.9 * t^2 = 25.0 * t - 4.9 * t^2

    15 = 25.0 * t

    t = 15 / 25 = 0.6 s

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