What force must be applied to keep a concrete slab weighing 720 kg under water?

Initial data: m (mass of a given concrete slab) = 720 kg.

Reference data: ρb (average density of heavy concrete) = 2350 kg / m3; g (acceleration due to gravity) ≈ 10 m / s2; ρw (density of water into which the concrete slab was immersed) = 1000 kg / m3.

The force required to hold the concrete slab is determined by the formula: Fр = Fт – Fa = m * g – ρw * g * V = m * g – ρw * g * m / ρb = m * g * (1 – ρw / ρb ).

Calculation: Fр = 720 * 10 * (1 – 1000/2350) ≈ 4136 N.

Answer: It is necessary to apply force 4136 N.