# What work must be done to lift a stone weighing 10 kg from the bottom of the lake to the surface of the water?

**What work must be done to lift a stone weighing 10 kg from the bottom of the lake to the surface of the water? The depth of the lake is 3 m, the density of the stone is 2500 kg / m3. Neglect water resistance.**

Given:

m = 10 kilograms is the mass of a stone lying at the depth of the lake;

h = 3 meters – the depth of the lake;

ro = 2500 kg / m ^ 3 is the density of the stone;

ro1 = 1000 kg / m ^ 3 – water density;

g = 10 N / kg – acceleration of gravity.

It is required to determine the work A (Joule) that needs to be done to raise the stone.

Let’s find the volume of the stone:

V = m / ro = 10/2500 = 0.004 m ^ 3.

Then, the buoyancy force acting on the stone is equal to:

Fa = ro1 * g * V = 1000 * 10 * 0.004 = 40 Newtons.

The weight of a stone in water will be equal to:

P = F gravity – Fa = m * g – Fa = 10 * 10 – 40 = 100 – 40 = 60 Newtons.

To lift a stone, you need to apply to it a force equal to its weight, that is: F = P = 60 Newtons.

Then the work will be equal to:

A = F * h = 60 * 3 = 180 Joules.

Answer: you need to do work equal to 180 Joules.