Inertia & Amount Of Substance

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Inertia is the property of a mass which resists change from its state of rest or motion.

  • The inertia of an object refers to the reluctance of the object to start moving if it is stationary in the first instance or the reluctance of the object to stop moving if it is moving in the first instance.
  • When a body of matter is stationary, it needs a force to make it start moving. The bigger the mass, the bigger the force needed. We say that masses have inertia: a reluctance to start moving.


The mass (m) of a body of matter is quantitative measure of its inertia, i.e., its resistance to a change in the state of rest or motion of the body, when a force is applied.

  • SI unit of mass is the kilogram (kg). It is a scalar quantity.
  • The greater the mass of a body, the smaller the rate of change in motion.


Matter is the material substance that constitutes the observable universe and, together with energy, forms the basis of all objective phenomena. The basic building blocks of matter are atoms. The atoms themselves comprise of nucleus and electrons.

Amount Of Substance

Although mass is defined in terms of inertia, it is conventionally interpreted as:

The mass (m) of a body of matter is a measure of its amount of substance in the body.

  • Under ordinary circumstances, matter does not change. Hence, the amount of substance in the body can be assumed to be quantitatively equal to its mass. If the amount of substance divides itself, we can assume that its mass also halved.
  • Mass can be measured with a beam balance or an electronic balance.

Note: As you might recall in Base quantities, the base SI unit for amount of substance is mol.

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