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11 October, 16:12

A rock dropped from a 20 meter bridge falls into the river below. How long did it take to reach the water? What velocity did it hit the water with.

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  1. 11 October, 18:00
    0
    2 s, - 20 m/s

    Explanation:

    Given:

    y₀ = 20 m

    y = 0 m

    v₀ = 0 m/s

    a = - 9.8 m/s²

    Find: t and v

    y = y₀ + v₀ t + ½ at²

    0 = 20 + 0 + ½ (-9.8) t²

    0 = 20 - 4.9 t²

    t ≈ 2 s

    v² = v₀² + 2a (y - y₀)

    v² = 0 + 2 (-9.8) (0 - 20)

    v ≈ ±20 m/s

    Since the rock is falling, v = - 20 m/s.
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