Density and Specific Gravity

len Alfred Ajibola - Tue, 10th September, 2019 @ 16:25: PM

Topics in Physics

Density and Specific Gravity Force and Motion - Newton's Laws of Motion Motion, Speed, Velocity and Acceleration Heat Transfer, Conduction, Convection and Radiation Concept of Heat and Temperature Gravity explained with some fun facts Zeroth Law of Thermodynamics - Thermal Equilibrium Explanation, Similarities and Differences between Fundamental and Derived Units Physics - Basic and Fundamental Physics Concepts

Academic Questions in Physics

Please check out our Test Your Knowledge page to see all Questions and Answers

Consider this expression below:

A body X exerts a force FX on another body Y. In return, body Y exert a force FY back on body X. The forces FX and FY are equal in magnitude but opposite in direction.

Putting the above expression into equation:

FX = -FY

Where FX is the action force

FY is the reaction Force

The above expression relates to _____.

  • A. Theory of Force
  • B. Force Postulate
  • C. Einstein's proposal on Relativity
  • D. Newton's 1st law of motion
  • E. Newton's 2nd law of motion
  • F. Newton's 3rd law of motion

The tendency of a body to remain in a state of rest or in uniform motion is referred to as _____.

  • A. tension
  • B. uniform acceleration
  • C. constant acceleration
  • D. deceleration
  • E. inertia
  • F. velocity

A change in the position of an object with reference to time is best described as _____.

  • A. momentum
  • B. displacement
  • C. velocity
  • D. acceleration
  • E. direction
  • F. motion

The saying "what goes up on earth must come down" can be best attributed to _____.

  • A. force
  • B. motion
  • C. mass
  • D. acceleration
  • E. average velocity
  • F. constant velocity

We have _____ laws of Thermodynamics.

  • A. 1
  • B. 2
  • C. 3
  • D. 4
  • E. 5
  • D. 6

Which Scientist successfully discovered the theory of special relativity?

  • A. Albert Einstein
  • B. Isaac Newton
  • C. Galileo Galilei
  • D. Neils Bohr
  • E. Ernest Rutherford
  • F. Marie Curie

The scientific study of matter and the movement of matter through space and time is a concept of _____.

  • A. Biology
  • B. Chemistry
  • C. Physics
  • D. Basic Science
  • E. Geography
  • F. Physiology

10oF is associated with _____.

  1. temperature
  2. pressure
  3. atmospheric pressure
  4. rainfall
  5. direction of wind
  6. speed of wind


Density has got a great deal to do with the mass/weight of substances. This is so because equal volumes of different substance have got different masses and weight; for instance; 10 gram of iron will be heavier (have a higher density) when compared to 10 gram of wood (which has a lower density). Similarly, 100ml of water will have a higher density than 100ml of oil; and for this reason, oil floats on water. Please read on Separation of mixtures (water and oil) here.

Note: What makes the difference in the density of different materials is as a result of the different atoms that make them up; for instance; water (H2O), which is made up of (Hydrogen and Oxygen in the ratio of 2:1) is different the constituents making up oil.

By Definition, Density is the mass of a unit volume of a material or substance.

You can read more on mass here.

Note: The principle of density was determined by a Greek scientist named Archimedes.

The formula for density is:
ρ = M/V
ρ = density
M = mass
V = volume
Note: Density can also be expressed by the Greek letters "rho"

The Standard International (SI) Unit for Density is Kilogram per Cubic Metre (Kg/m3). Below are some substances with their varying densities:


 DENSITY (Kg/m3)



 Air (1atm and 20oC)


 Bamboo Wood




 Methylated Spirit




 Paraffin Wax




 Water (Fresh Water)


 Sea Water (Saline/Salt Water)


 Blood. Please read on the Red Blood Cells here




 Aluminium  (Al)


 Zinc (Zn)


 Iron (Fe)


 Copper (Cu)


 Lead (Pb)


 Mercury (Hg)


 Gold (Au). Please read interesting facts on Gold


 Platinum (Pt)


Notice from the table that helium has a lower density than air and that's one reason why it's used in filling balloons.

Having an understanding of density offers a convenient means of deriving the mass of a body from its volume. On the flip side, when we know the mass of a body, we can derive it's density.

Below are the derivation steps

Mass =  volume x density (M = Vd)

Volume = mass divided by the density (V = M/d).

Note: To get the weight of a body, we will need to multiply its mass by the acceleration of gravity.

Please read more on Gravity here.

A common application of density is seen in the way at which different materials interact with each other. For instance, a wood may float on water while an iron may sink in it. Also, balloons will float in air because it's filled with helium gas which is much lighter (has a lower density) than air.


Relative Density / Specific Gravity

Relative density involves the density of one substance in relation to that of water (at 4oC). it's also called specific gravity.

The Relative Density (R.D) of a material is the ratio of it's density (the material) to that of water at 4oC.

By principle, an object with a specific gravity/relative density of less than 1 will float on water, while that with value greater than 1 will sink in water.

Below are the formulas for calculating relative density.

Relative Density; R.D = Mass of Substance/Mass of Water at 4oC

R.D = Weight of Substance/Weight of Water

R.D = Density of Substance/Density of Water at 4oC

R.D = ρsubstance / ρreference (water at 4oC)

Where ρ = Density

Note: Relative Density has no Standard International (SI) unit.


The table below shows the relative density of some substances:



 Balsa Wood


 Oak Wood


 Ethanol (C2H5OH)


 Liquid Paraffin


 Olive Oil


 Coconut Oil




 Water (Fresh Water)


 Salt Water (Reef Aquarium)





 1.9 – 2.3

 Common  Salt (NaCl)


 Aluminium (Al)




 Iron  (Fe)


 Copper (Cu)


 Lead (Pb)


 Gold (Au)


 Osmium (Os)


THANKS FOR READING - Please Help Share!


Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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