Question

Which of the following physical quantities has no unit?
A. Relative density
B. Density
C. Pressure
D. Thrust

Answer 1

Hint: Learn the definition of density, relative density, pressure and thrust and find their units. If we divide two same units then the resultant will be unit less. For example, the ratio of $\dfrac{density}{density}$ is unit less. The quantity which has no unit is a constant term.

Complete step by step answer:
Relative density is the ratio of density of substance and density of reference substance.
\[Relative\text{ }density=\dfrac{density\text{ }of\text{ }substance}{density\text{ }of\text{ }reference\text{ }substance}\]
Here, we can observe from the above formula that relative density is ratio of density and density. So it will have no unit.
If a substance’s relative density is less than one than it is less dense than the reference and if greater than one ten it is denser than the reference. If the relative density is exactly one then the density of substance is equal to density of reference substance.

Additional Information:
Density – it is the ratio of mass per unit volume. Unit of density is $kg/{{m}^{3}}$
Pressure is force per unit area. Unit of pressure is Pascal.
Thrust is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. Unit of thrust is Newton.

Note: Relative density is also known as specific gravity. If there is a term “specific” it means that quantity has no unit. Specific gravity for liquids is nearly always measured with respect to water. If the reference substance is water and the relative density of the substance is greater than one then it will sink in water.