As per physics, the mass of an object is determined by the number and type of atoms or molecules it contains and also by the energy involved in binding it together. So, mass accounts for the amount of matter in an object. The mass of an object determines its acceleration when a force is applied to it. This process is called inertia. Newton, in his second law of motion, said that if a single force F is applied to a body of fixed mass m, then its acceleration is given by F/m. The mass of the body also tells the degree to which it is affected by a gravitational field. The S.I unit of mass is kg (kilogram).

There are several phenomena used to measure mass. *Inertial mass *measures an object’s resistance when it is being accelerated by a force. *Active gravitational mass *tells about the gravitational force exerted by an object. *Passive gravitational mass* of an object measures the gravitational force experienced by it in a known gravitational field. *Mass Energy* relationship explains the total amount of energy contained within a body (E=mc^{2}).

**Let us go through some basic facts about Mass:**

- Mass is not the same as weight of a body. Weight is dependent on the gravitational force being faced by an object. The weight of the objects is more at sea level as it is close to the center of the Earth than on a mountain which is away from the Earth’s center. The gravitational force of Earth is stronger at sea level.
- Mass always remains constant but the weight of a body differs with the force of gravity. So, mass is an absolute term that is always used by the scientists to tell how heavy an object is.
- Mass of a body remains same even when taken to the Moon. The weight of a person on the Moon is equivalent to one-sixth of their weight on Earth because the gravitational force on Moon is one-sixth of it on Earth.
- The gravitational and inertial masses of a body are equal, says Einstein’s equivalence principle of relativity. It means that at the same location equal (inertial) masses also have equal weights.
- As per the special theory of relativity, mass does not remain stringently constant when the body accelerates at very great speeds, according to the formula, m=m0 / sqrt (1-v2/c2), where m0 is the mass of the body at rest, v is its velocity and c is the speed of light in vacuum. This theory led to the formation of equation E=mc
^{2 }which accommodates mass as a form of energy. - In year 1785, Antoine Laurent Lavoisier, introduced the Law of Conservation of Mass according to which, in a chemical reaction, matter (mass) is neither created nor destroyed.
- The photon has the smallest mass equivalent to 1.6726*10
^{-27}kg. - The mass of the Earth is 5.97219 × 10
^{24}kg and that of the known universe is believed to be 10^{50}tons or 10^{53}kg. - Mass is different from size or volume. The type of atoms or molecules along with the density tells the mass of the body. A balloon filled with helium will have quite less mass than a similar sized ball made of solid gold.
- The heavier the object, the more mass it has. One needs to apply stronger brakes to stop a truck than to stop a bicycle. This is because a truck has more mass than a bicycle.
- Density is the amount of mass in a certain space and measured in grams per cubic centimeter. The lightest solids, silica aero gels, have a density of 0.005 g/cm
^{3}. - Hydrogen is the lightest gas. Osmium is the densest solid having density of 22.59 g/cm
^{3}. A neutron star, the densest and smallest star, has an unbelievable mass of about two times that of Sun.^{ }

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