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4 June, 16:10

As shown, a load of mass 10 kg is situated on a piston of diameter D1 = 140 mm. The piston rides on a reservoir of oil of depth h1 = 42 mm and specific gravity S = 0.8. The reservoir is connected to a round tube of diameter D2 = 5 mm and oil rises in the tube to height h2. Find h2. Assume the oil in the tube is open to atmosphere and neglect the mass of the piston.

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  1. 4 June, 18:11
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    165 mm

    Explanation:

    The mass on the piston will apply a pressure on the oil. This is:

    p = f / A

    The force is the weight of the mass

    f = m * a

    Where a in the acceleration of gravity

    A is the area of the piston

    A = π/4 * D1^2

    Then:

    p = m * a / (π/4 * D1^2)

    The height the oil will raise is the heignt of a colum that would create that same pressure at its base:

    p = f / A

    The weight of the column is:

    f = m * a

    The mass of the column is its volume multiplied by its specific gravity

    m = V * S

    The volume is the base are by the height

    V = A * h

    Then:

    p = A * h * S * a / A

    We cancel the areas:

    p = h * S * a

    Now we equate the pressures form the piston and the pil column:

    m * a / (π/4 * D1^2) = h * S * a

    We simplify the acceleration of gravity

    m / (π/4 * D1^2) = h * S

    Rearranging:

    h = m / (π/4 * D1^2 * S)

    Now, h is the heigth above the interface between the piston and the oil, this is at h1 = 42 mm. The total height is

    h2 = h + h1

    h2 = h1 + m / (π/4 * D1^2 * S)

    h2 = 0.042 + 10 / (π/4 * 0.14^2 * 0.8) = 0.165 m = 165 mm
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