Data: V (volume of the lifted granite slab) = 4 m3; h (lifting height) = 2 m; η (efficiency of the mechanism with which the plate was lifted) = 60% (0.6).
Constants: g (acceleration due to gravity) ≈ 10 m / s2; ρg (density of granite) = 2600 kg / m3.
The work that was done when lifting the granite slab is determined by the formula: η = Ap / Az, whence Az = Ap / η = ΔEn / η = m * g * h / η = ρg * V * g * h / η.
Calculation: Az = 2600 * 4 * 10 * 2 / 0.6 ≈ 347 kJ.
Answer: It is necessary to do a work of 347 kJ.
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