What force must be applied to lift a stone weighing 20 kg under water, the volume of which is 15000 cm³.

m = 20 kilograms is the mass of a stone lying under water;

V = 15000 cubic centimeters – the volume of the stone;

ro = 1000 kilograms / cubic meter – density of water.

It is required to determine F (Newton) – what force must be applied to lift the stone.

Let’s translate the units of measurement of volume in the SI system:

V = 15000 cm3 = 15000 * 10-6 = 15000/1000000 = 0.015 m3.

We will assume that the stone must be lifted evenly at a constant speed. Then, according to Newton’s first law:

F = F gravity – Farchimedes;

F = m * g – ro * V * g, where g = 10 Newton / kilogram (approximate value);

F = g * (m – ro * V);

F = 10 * (20 – 1000 * 0.015) = 10 * (20 – 15) = 10 * 5 = 50 Newtons.

Answer: to lift a stone out of the water, you need to apply a force equal to 50 Newtons.