An object fully submerged in water has volume 0.0025 m^3. If the density of water is 1000 kg/m^3 and g = 9.8 m/s^2, what is the buoyant force?

• A. 2.45 N
• B. 24.5 N
• C. 245 N
• D. 2450 N

Answer: (B)

Buoyant force is the weight of the fluid that the object displaces. Using Archimedes’ principle, F_b = ρ V g. Here, ρ = 1000 kg/m^3 and V = 0.0025 m^3, so the displaced water has mass m = ρV = 1000 × 0.0025 = 2.5 kg. Its weight is m g = 2.5 × 9.8 = 24.5 N. Therefore, the buoyant force is 24.5 newtons. Quick cross-check: this comes directly from multiplying the displaced mass by gravity, so any other result would require a different volume or density.