In physics, fluid dynamics is a critical concept determining whether an object will float or sink in water. Specific gravity is the ratio of the density of the object and a reference substance. Specific gravity can tell us if an object floats or sinks in the reference object based on its value. In this article, we will be closely looking at the concept of specific gravity.
Specific Gravity Explained
We define specific gravity as the ratio of the object’s density to the ratio of the water’s density at a specified temperature and pressure. It is customary to use water’s density at 4oC as a reference point because the water at this point has the highest density of 1000 kg/m3.
The specific gravity is derived by dividing the material’s density by the water’s density at 4 degrees Celsius. For the calculation purpose, the material’s density and water’s density must be expressed in the same units.
In the table below, we have expressed the density of water at 4oC in different units.
| Density of Water in Various Units
● 1000 kilograms per cubic meter ● 62.43 pounds per cubic foot ● 1 gram per cubic centimetre ● 0.036 pounds per cubic inch |
Factors Affecting the Specific Gravity of a Substance
Variation in temperature and pressure affect an object’s density, thereby changing the object’s specific gravity. For this reason, standard pressure and temperature are used to calculate specific gravity.
What are some Specific Gravity applications?
Specific gravity has a wide range of applications, and some of these applications are listed below:
- A gemstone’s value can be determined with the help of specific gravity’s value. By comparing the specific gravity of the gem whose purity has to be measured with the specific gravity of the high purity gem, one can determine the purity of a gemstone.
- To determine the mineral content of a given rock, mineralogists and geologists use specific gravity.
- Urinary specific gravity gives us a degree of the concentration of solutes in the urine.
Solved Examples of Specific Gravity
What is the specific gravity of Iron?
The density of iron is 7850 kg/m3. Therefore, the specific gravity of iron is calculated to be
Specific gravity of iron = (7850 kg/m3)/(1000 kg/m3) = 7.85
The specific gravity of iron is 7.85.
It should be noted that specific gravity is a dimensionless quantity.
What is the specific gravity of ice?
The density of the ice is 920 kg/m3.
Now calculating the specific gravity of ice, we get
Specific gravity of ice = (920 kg/m3)/(1000 kg/m3) = 0.92
The specific gravity of ice is 0.92
Centre of Gravity vs Specific Gravity
People often mistake the centre of gravity and specific gravity to be the same. In this section, we will be discussing what the centre of gravity is so that the distinction between these two terms are clear to you.
The centre of gravity is a theoretical point in a body where the total weight of the body is assumed to be concentrated. The centre of gravity is useful in predicting the behaviour of the body when acted upon by gravity. The lower the centre of gravity is, the more stable the object is. The higher the centre of gravity, the more likely it is that it topples over. The centre of gravity in the human body does not remain constant as it changes with every new position of the limbs and body.
