# Thread: What is Science's basic definition of mass?

1. I have a very poor knowledge of what science is and can be? So if this is for me what is rather stupid you know please excuse it.

Mass is how much matter something is? However if matter can be made of sub- atomic particles say and sub- atomic particles can have different atomic masses then inst mass not the "count" of sub-atomic particles as a first idea so much as the space all those sub atomic particles "individually" and together take up? So that density means nothing as the basic idea how much matter there is based on that it's not so much the "count" of sub-atomic particles that makes up so much as their atomic mass that determines mass?

So density only tells us if an object takes up more space or mass as a comparative of other objects with lesser or more density but which have sub-atomic particles of the same atomic mass.  2.

3. Originally Posted by LotusTiger
I have a very poor knowledge of what science is and can be? So if this is for me what is rather stupid you know please excuse it.

Mass is how much matter something is? However if matter can be made of sub- atomic particles say and sub- atomic particles can have different atomic masses then inst mass not the "count" of sub-atomic particles as a first idea so much as the space all those sub atomic particles "individually" and together take up? So that density means nothing as the basic idea how much matter there is based on that it's not so much the "count" of sub-atomic particles that makes up so much as their atomic mass that determines mass?

So density only tells us if an object takes up more space or mass as a comparative of other objects with lesser or more density but which have sub-atomic particles of the same atomic mass.
Mass is not well-defined. There are several valid notions of mass.

In Newtonian mechanics you have the relationship which for constant mass systems, like particles reduces to which defines inertial mass given knowledge of force, or force given knowledge of inertial mass.

You also have the law of universal gravitation which gives the gravitational force in terms if the gravitational masses of two bodies.

There is no a priori reason why gravitational and inertial mass need be equal. But it is a happy coincidence, born out by many experiments, that they are indeed equal.

When you go on to relativity there are three common versions of mass: rest mass, relativistic mass and invariant mass.

Rest mass is the mass of a body that has zero velocity in the frame of the observer. In relativity mass is dependent on the reference frame in which it is measured.

If the body is in motion at speed relative to an observer then the inertia relative to the observer is reflected in an equation similar to that of Newton where is the relativistic mass and In the case of a system of particles whizzing about, as with a block of solid matter composed of atoms and molecules, the mass that you would measure on a laboratory balance includes relativistic effects, but a choice of convenient reference frame is needed. The obvious, and correct, choice is the frame of the laboratory. That is also the frame in which the net momentum of all of the particles in the block is zero, the center-of-mass frame. So, the sum of the relativistic masses in this frame is the mass measured on a laboratory balance, and is called invariant mass. it is also used by particle physicists who study several high-speed particles simultaneously.

The mass of an ordinary block of matter is the sum of the invariant masses of the sub-atomic particles of which it is composed. This is also the total energy/ divided by in accordance with Einstein's equation which holds with denoting relativistic mass for a single particle or invariant mass for a system.  4. What does the G the gravitational constant mean in the law of universal gravitation. I understand that the force between two bodies is equal to their masses and inversely proportional to the square distance between then.(r) This is the law of universal gravitation.

Is G (gravitational constant) the set gravitational pull at that point in a gravitational field of a body depending on it's mass. In other words does G stand for the fact that a body of a particular mass will exert a specific gravitational pull at a particular point in their gravitation field. This all depending on the mass.

Whereby this fact Gravitational Constant is present in both bodies where we take into account universal gravitation and thus we can calculate the force between those two bodies in their course to each other.

So in other words is the Gravitational Constant the force at that point in a masses gravitational field or at least the idea that a mass will have a particular gravitational pull at a particular point in it's gravitational field.

Also what determines a bodies passive gravitational mass. Is the passive gravitational mass determined only by it's mass? Also do we say a body has that measurement of passive gravitational mass at that point in a gravitational field in that it will accelerate that much at that point of a gravitational field of that "strength" telling us it's mass I guess. Or rather does it not apply as a value in such a way but rather that we say a body has a passive gravitational mass of that value in that this value is not so much how much it will accelerate at that point of a gravitational field of that strength but rather it means that we say that body has such a specific value in that if it were to accelerate that much at a point in a gravitational field at that strength then it would have x value but this value does not denote the specific value of how much it accelerates(N) at a point in a gravitational field of that strength but rather that that value(N) determines the value of a bodies passive gravitational mass. In other words that value(N) determines it but isn't it.  5. Every 'type' of mass is easier defined by its equivalent energy. And, correct me if I'm wrong DrR, but energy is well defined.  6. Originally Posted by MigL
Every 'type' of mass is easier defined by its equivalent energy. And, correct me if I'm wrong DrR, but energy is well defined.
Nothing is as well-defined as is desirable. establishes that mass and energy are really the same thing, again depending on what you mean by "mass". but energy is very much dependent on the reference frame.

You can adopt a very abstract approach and say that energy is the conserved Neother current that comes from time translation symmetry. Some people find that enlightening.

But then you have to contend with general relativity, and in GR conservation of energy is a very subtle thing. So is "gravitational energy".

Bottom line: relating mass to energy is important to the physics, but doesn't help establish a clean definition of either mass or energy.

Physics is not an axiomatic science. One of the original Hilbert problems was to put physics on an axiomatic basis. It was never accomplished. That problem was proposed before the invention of quantum mechanics and relativity. So physics is now farther from being axiomatic than when David Hilbert proposed the problem.

Mathematics has the luxury of not having to deal with reality.

With physics the concepts tend to evolve along with depth of understanding. With mass and energy it is probably best to start with and for an understanding of mass, which yields operational definitions and measurement standards. Work is then force x distance and energy is associated to work. Beyond that understanding comes in steps as one learns about electromagnetism, relativity and quantum theory. That understanding is quite likely never complete.  7. DrRocket I am impressed

Every 'type' of mass is easier defined by its equivalent energy. And, correct me if I'm wrong DrR, but energy is well defined.
Do you define energy as E=mc^2 or something more?  8. Originally Posted by Eleven
DrRocket I am impressed

Every 'type' of mass is easier defined by its equivalent energy. And, correct me if I'm wrong DrR, but energy is well defined.
Do you define energy as E=mc^2 or something more?
E=mc^2 expresses the total energy of a body. But in terms of a definition it begs the question "What is m ?'. There is an unavoidable circularity.  9. Thank You. There is something more...  Bookmarks
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