# Mass

## Explanation

Mass of a system or a particle is the most interesting concept in physics that still doesn’t have a valid explanation. A broad explanation for mass is that it is a resistance of an object to any acceleration, according to the classical physics. It's generally known as the inertial mass or inertia and is represented as ${\vec{F}=m\vec{a}}$.

However, in special relativity the term relativistic mass comes into play, as mass is interchanged to energy and vice versa. This can be understood from the mass–energy equivalence formula $e=mc^2$. According to this, mass can be defined as an intrinsic property of the system of interest that arises from the kinetic and binding energies of the quarks along with the potential energy of the gluon field, which together make up the nucleons. Along with the interaction of other fields, mass manifests itself from the energy. And according to general relativity the presence of mass warps spacetime too. Now this ability of mass to curve spacetime is called as the gravitational mass.

However, according to the equivalence principle, the inertial mass and gravitational mass are the same. Which just means that when you are in a closed container and subjected to a constant gravitational field and then a constant acceleration, you cannot distinguish between either of them. The mass is generally expressed in terms of its SI unit as kilogram.

Weight is simply the force acting on an object of a certain mass that is subjected to gravity. On the other hand, mass is an intrinsic property fundamental to the object as explained above. Weight is related to mass as a multiple of local acceleration due to the gravity of the system the object is in – ${\vec{W}=m\vec{g}}$. Weight is measured in its SI unit as newton.