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15 June, 14:01

A brick is thrown vertically upward with an initial speed of 3.00 m/s from the roof of a building. If the building is 78.4 m tall, how much time passes before the brick lands on the ground?

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Answers (2)
  1. 15 June, 14:15
    0
    0.918 sec

    Explanation:

    time of flight:

    t=u²/g

    =3²/9.8=0.918sec
  2. 15 June, 14:29
    0
    4.32 seconds

    Explanation:

    Given:

    y = 0 m

    y₀ = 78.4 m

    v₀ = 3.00 m/s

    a = - 9.8 m/s²

    Find: t

    y = y₀ + v₀ t + ½ at²

    0 = 78.4 + 3.00 t - 4.9 t²

    4.9t² - 3t - 78.4 = 0

    Solve with quadratic formula:

    t = [ 3 ± √ (9 - 4 (4.9) (-78.4)) ] / 9.8

    t ≈ - 3.71, 4.32

    Since t must be positive, t = 4.32. So it takes 4.32 seconds for the brick to land.
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