Buoyant Force — Life Jacket (Worked Example)

Given

• V = 0.015 m³

• ρ = 1025 kg/m³

• g = 9.8 m/s²

Find

The buoyant force Fb.

Principle

Fb = ρVg for the displaced seawater — this is what keeps a swimmer's head above water.

Formula

Solution

1. Substitute: $F_b = (1025)(0.015)(9.8)$

2. Note: only the foam's volume displaces water — the wearer's body is supported separately

3. Multiply: $F_b = 150.7\ \mathrm{N}$

Unit check

kg/m³ × m³ × m/s² = N — correct.

What this means physically

151 N — about 15 kg of lift, enough to support an adult's head and shoulders. The foam's own weight is negligible, so nearly all of this is useful buoyancy.