These tasks: V (volume of granite cobblestone) = 0.004 m3.
Reference values: g (acceleration due to gravity) ≈ 10 m / s2; according to the condition ρw (water density in the lake) = 1000 kg / m3.
The buoyancy force acting on a granite cobblestone is calculated by the formula: Fa = ρw * g * V.
Let’s calculate: Fa = 1000 * 10 * 0.004 = 40 N.
Answer: A buoyant force of 40 N acts on a granite cobblestone at the bottom of the lake.
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