A granite slab 1.2 m long, 40 cm wide and 25 cm thick sits at the bottom of the river. How much force must be applied to lift it?

Given: l (length of the granite slab) = 1.2 m; b (slab width) = 40 cm (in SI b = 0.4 m); s (thickness) = 25 cm (s = 0.25 m).

Constants: g (acceleration due to gravity) ≈ 10 m / s2; ρw (water density) = 1000 kg / m3; ρg (density of granite) = 2600 kg / m3.

The force required to lift the slab from the bottom of the river is determined by the formula: F = Ft – Fa = m * g – ρw * g * V = ρg * V * g – ρw * g * V = V * g * (ρg – ρw ) = l * b * s * g * (ρg – ρv).

Calculation: F = 1.2 * 0.4 * 0.25 * 10 * (2600 – 1000) = 1920 N (1.92 kN).