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A.)Specific Gravity

B.)Density

C.)Gravity

D.)None of these

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The density $\rho = \dfrac{mass}{volume}$ , often measured in $kg\;m^{-3}$ or $g\;cm^{-3}$

Let us first try and understand what mass per unit volume means.

We know that the mass of an object is the amount of matter it contains, and numerically this amount of matter is called its inertia. The SI unit of mass is kilogram (kg)

The volume of an object is the amount of space it occupies in three dimensions, (since the two-dimensional space it occupies would be just its area). Volume can also be understood as the capacity of a container to hold matter. The SI unit of volume is cubic metre ($m^3$) which is equivalent to 1000 litres.

Now, based on how molecules are arranged (i.e., if they are tightly packed or loosely packed) the amount of matter that the volume of an object can hold varies. If the molecules are very close together in a volume and are compactly packed they form “dense” materials, whereas if they are loosely packed the material will be relatively “less dense”. From this we can deduce that the amount of matter packed into the given volume will determine the “density” of the material.

So, quantitatively this can be expressed as:

$$density\;\rho = \dfrac{mass}{volume} kg\;m^{-3}$$

If we take a unit volume, i.e., $1 m^3$ then:

$$density\;\rho = \dfrac{mass\; kg}{1\;m^{3}} = mass\;per\;unit\;volume$$

An object’s mass also determines the strength of its gravitational attraction to other bodies (which is called its weight). Its SI unit is newton (N). It is given by Weight w = mass $\times$ gravity, where gravity on the surface of the earth is given as $9.8 ms^{-2}$. Do not confuse mass with weight as mass is the amount of matter a body contains while weight is the gravitational force acting on that body.

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