### Theory:

Why does the wood float and stone sink in water?
Wood float in water because they are less dense than water. It floats because it weighs less than amount of water it would have to push out from the water if it sank. Wood is less dense than water, and they float; rocks are more dense, so they sink.
Density of a substance is defined as the mass of the substance contained in unit volume $$1\ m^3$$.
The lightness or heaviness of a body is due to density.
Example:
In an oil spill in the ocean, the oil rises to the top because it is less dense than water, creating an oil slick on the surface of the ocean. If the mass of a substance is denoted as M and whose volume is denoted as V, then, the equation for density is given as
$\mathit{Density}\phantom{\rule{0.147em}{0ex}}\left(D\right) =\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)}{\mathit{Volume}\phantom{\rule{0.147em}{0ex}}\left(V\right)}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}D\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\frac{M}{V}$
The density of SI unit is $$kg / m^3$$ and then the density of CGS (centimetre Gram Second) unit is $$g / cm^3$$

Density of different materials:

The different material has different density due to their nature. The denser is a material which as a higher density and lighter is a material which as a lower density.
The density of a different materials shown below.
 S.no Material Density 1 Air $$1.2$$ 2 Kerosene $$800$$ 3 Water $$1,000$$ 4 Mecury $$13,600$$ 5 Wood $$770$$ 6 Aluminium $$2700$$ 7 Iron $$7,800$$ 8 Copper $$8,900$$ 9 Silver $$10,500$$ 10 Gold $$19,300$$

Relationship between the density, mass and volume: $\begin{array}{l}\mathit{Density}\phantom{\rule{0.147em}{0ex}}\left(D\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)}{\phantom{\rule{0.147em}{0ex}}\mathit{Volume}\left(V\right)}\\ \\ \mathit{Mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathit{Density}\phantom{\rule{0.147em}{0ex}}\left(D\right)\phantom{\rule{0.147em}{0ex}}×\phantom{\rule{0.147em}{0ex}}\mathit{Volume}\phantom{\rule{0.147em}{0ex}}\left(V\right)\\ \\ \mathit{Volume}\phantom{\rule{0.147em}{0ex}}\left(V\right)\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\frac{\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)}{\phantom{\rule{0.147em}{0ex}}\mathit{Density}\phantom{\rule{0.147em}{0ex}}\left(D\right)}\end{array}$
Example:
Let us work out a problem:

A sphere is made from iron whose mass is $$3000$$ $$kg$$. If the density of iron is $$7800$$ $$kg/m^3$$, find the volume of the sphere.

$\begin{array}{l}\mathit{Given}:\\ \\ \mathit{Volume}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}3000\phantom{\rule{0.147em}{0ex}}\mathit{kg}\\ \\ \mathit{Density}\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}7800\phantom{\rule{0.147em}{0ex}}\mathit{kg}/{m}^{3}\\ \\ \mathit{Volume}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{sphere}:\\ \\ \mathit{Volume}\phantom{\rule{0.147em}{0ex}}\left(V\right)\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\left(M\right)\phantom{\rule{0.147em}{0ex}}/\phantom{\rule{0.147em}{0ex}}\mathit{Density}\phantom{\rule{0.147em}{0ex}}\left(D\right)\\ \\ =\phantom{\rule{0.147em}{0ex}}3000\phantom{\rule{0.147em}{0ex}}/\phantom{\rule{0.147em}{0ex}}7800\\ \\ \phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}30\phantom{\rule{0.147em}{0ex}}/\phantom{\rule{0.147em}{0ex}}78\\ \\ =\phantom{\rule{0.147em}{0ex}}0.38\phantom{\rule{0.147em}{0ex}}{m}^{3}\end{array}$
This is the way to find the volume of a solid material