In Class 9 Physics (Gravitation and Fluids), to determine if a heavy iron block or a light wooden block will float or sink in water, we use a concept called Relative Density.
Formula: Density of Substance ÷ Density of Water.
Unit: No unit (it is a pure ratio).
Density of Water: 1000 kg/m³.
Density is simply mass per unit volume ($Density = Mass / Volume$). However, Relative Density is the ratio of the density of a specific substance compared to the density of water.
Water is chosen as the standard reference because its density is exactly $1000 , kg/m^3$ (or $1 , g/cm^3$) at 4°C.
Relative Density = $\frac{\text{Density of a Substance}}{\text{Density of Water}}$
Important Note: Because relative density is a ratio of two identical quantities (density divided by density), the units cancel out. Therefore, Relative density has NO unit.
Relative density quickly tells us if an object will float or sink in water:
The formula for relative density is the Density of a substance divided by the Density of water. Since it is a ratio, it does not have any unit.
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