Why is mass constant?

Question

Why is mass constant?

Answer 1

Because it is the mass of the particles composing the matter which don't change in normal conditions you know chemical and nuclear reactions cause changes in the structure of molucles , atoms and even nucleuses this lead to increasing or decreasing mass but in normal conditions (away from heat , pressure etc.) mass remains the same.

Answer 2

Asking why in modern physics is frustrating, and usually gets you nowhere. Usual response is that it is an inherent feature of matter, and is used as the definition of matter.

wikipedia:

Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of how much matter there is in an object. Mass is a central concept of classical mechanics and related subjects, and there are several definitions of mass within the framework of relativistic kinematics (see mass in special relativity and mass in General Relativity). In the theory of relativity, the quantity invariant mass, which in concept is close to the classical idea of mass, does not vary between single observers in different reference frames.

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Answer 3

Well the actual question is, what is mass ?

Mass can`t be measured directly, it can only be infered from its effects , like in the case of a weighing scale.

Here the weighing scale isn`t measuring your mass directly but its actualy meausuring your reaction force on it.This reading can change depending on where you are, i.e. if your on earth you weigh more than what you weigh on the moon, but the amount substance your made of doesn`t change, which is what your mass is, and not the reading on your weighing scale.

Answer 4

mass remains the same in all chemical, and  physical changes. The only time when mass changes is in nuclear processes, where mass is converted to energy, which can be calculated by the famous E=mc^2

All elemental atoms can be split/fused (fission and  fusion respectively) except for the Iron atom.