Question

Difficulty: MediumPascal's Principle and Hydraulics

In a hydraulic car lift system, a mechanic applies a downward force of 150 N150\text{ N} on a small input piston with a cross-sectional area of 5 cm25\text{ cm}^2. This force is transmitted through hydraulic fluid to a larger output piston that has a cross-sectional area of 100 cm2100\text{ cm}^2. Assuming ideal conditions, what is the maximum upward force exerted by the larger piston?

  1. A
    7.5 N
  2. B
    750 N
  3. 3,000 NAnswer
  4. D
    15,000 N

Answer

3,000 N
The correct answer is 3,000 N3,000\text{ N}. According to Pascal's Principle, the fluid pressure in the system remains constant (P=F1/A1=150/5=30 N/cm2P = F_1/A_1 = 150/5 = 30\text{ N/cm}^2). Multiplying this pressure by the area of the larger piston (30×10030 \times 100) yields an upward force of 3,000 N3,000\text{ N}. Alternatively, the mechanical advantage is the ratio of the two areas (100/5=20100/5 = 20). Multiplying the input force by this mechanical advantage (150×20150 \times 20) directly gives 3,000 N3,000\text{ N}.

Step-by-Step Solution

1
Calculate the pressure exerted by the input piston on the hydraulic fluid.
P=F1A1=150 N5 cm2=30 N/cm2P = \frac{F_1}{A_1} = \frac{150\text{ N}}{5\text{ cm}^2} = 30\text{ N/cm}^2
Pascal's Principle states that pressure applied to a confined fluid is transmitted undiminished in all directions.
2
Calculate the resulting upward force on the larger output piston.
F2=P×A2=30 N/cm2×100 cm2=3,000 NF_2 = P \times A_2 = 30\text{ N/cm}^2 \times 100\text{ cm}^2 = 3,000\text{ N}
The uniform pressure of 30 N/cm230\text{ N/cm}^2 acts over the entire 100 cm2100\text{ cm}^2 area of the larger piston, multiplying the original force.

Key Concept

Pascal's Principle and Hydraulic Mechanical Advantage
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