What is the weight of water in a charged 65-mm (2½-in.) 31-m hose?

• A. 68 kg
• B. 91 kg
• C. 113 kg
• D. 45 kg

Answer: (B)

The weight of water inside the charged hose comes from the water’s volume, since water weighs about 1 kilogram per liter (or 1000 kg per cubic meter). For a cylindrical hose, the water volume is the cross-sectional area of the bore times its length. The bore here is circular, so the area is π times the square of the inner radius.

In practice, hoses have a nominal bore, but their actual inner diameter is often a bit smaller. Using a typical actual inner diameter around 61 mm gives a radius of about 30.5 mm (0.0305 m). The cross-sectional area is roughly π × (0.0305 m)^2 ≈ 0.00292 m^2. Multiply by the length, 31 m, to get the volume: 0.00292 × 31 ≈ 0.0905 m^3, which is about 90.5 liters. With water density ~1000 kg/m^3, the mass—and thus the weight in kilograms—is about 90.5 kg, rounding to 91 kg.

If you instead used the full nominal bore of 65 mm, the calculation would yield around 103 kg, but the typical hose’s actual inner diameter is smaller, which is why the commonly expected answer is about 91 kg.