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Welcome to our Physics lesson on What is Density?, this is the third lesson of our suite of physics lessons covering the topic of Fluids. Density of Fluids, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
From definition of matter provided in the first paragraph, we can outline two physical features. They are:
Obviously, not all objects which have the same mass, occupy the same space. As an example, we can mention the situation described in the last question in the "Introduction" paragraph, i.e. when comparing 1 kg of iron and 1 kg of cotton. They have both the same mass but cotton occupies a much larger space than iron. For this reason, people tend to give a wrong answer when asked which object weighs more. In most cases, they chose iron as an option because they are confused by the fact that for the same volume, iron weighs more than cotton.
To avoid such confusion, it is better to introduce a new quantity that takes into consideration both mass and volume of objects. This quantity is known as "density" and it is calculated by the formula,
The above formula helps us give the definition of density. Thus, by definition,
"Density is the mass of a substance in the unit of volume."In formulae, we express the density by the letter ρ the mass by m and the volume by V. Therefore, the equation of density becomes
Obviously, the SI unit of density is [kg/m3] as mass is measured in [kg] and volume in [m3]. However, in many cases, especially when dealing with small amounts of matter, we often use another unit to express density. It is gram per cubic centimetre, i.e. [g/cm3]. The conversion factor between these two units is as follows:
In the above conversion, we have considered the fact that units of volume change 1000 by 1000. Thus, 1 m3 = 1000 dm3 = 1 000 000 cm3. Also, 1 kg = 1000 g.
A liquid sample has a mass of 360 g and it occupies 400 cm3 of space in its vessel. Calculate the density of liquid in g/cm3 and then convert it into kg/m3.
We have m = 320 g and V = 400 cm3. Thus,
When converted into kg/m3, the above result becomes
You have reached the end of Physics lesson 9.1.3 What is Density?. There are 5 lessons in this physics tutorial covering Fluids. Density of Fluids, you can access all the lessons from this tutorial below.
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