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Mass is one of the most core and complicated concepts in dynamics. I have tried many books but I still don't have a good idea of how the mass of any object is determined relative to another.

In The Science of Mechanics by Mach he says that two bodies isolated from all others are said to have same mass if they impart equal and opposite accelerations to each other. I think this assumes the third law.

Also it is not that easy to measure their accelerations.

Do we have any better and more useful definition of mass?

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  • $\begingroup$ Mach's answer is as good as you're going to get in classical mechanics (in which you do in general assume Newton's Laws hold). The question of whether this "inertial mass" is the same as the "gravitational mass" in general relativity for all types of matter is an open subject with a rich literature. $\endgroup$
    – rob
    Jan 5 '16 at 7:06
  • $\begingroup$ @rob But can we measure the instantaneous accelerations when they are varying with time so to assign mass.. $\endgroup$ Jan 5 '16 at 7:12
  • $\begingroup$ The measurement of mass is part of Newton's second law, and, yes, in physics you always measure everything relative to something else, unless it's a countable thing, in which case you count. $\endgroup$
    – CuriousOne
    Jan 5 '16 at 7:22
  • $\begingroup$ Can machs operational definition be used to assign masses or if it is too difficult ? $\endgroup$ Jan 5 '16 at 8:02
  • $\begingroup$ @John Forkosh I think in such universe we would have to take a third point as reference so we will have 2 diatances and also we cant take one of particles as reference as they would be a noninertial frame so we couldnt then use laws of motion. $\endgroup$ Jan 5 '16 at 8:07
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I suppose that you really mean the theory of Mechanics when asking about dynamics.

Let us clear the terms.

Mechanics is the branch of physics in which the basic physical units are developed. The logical sequence is from the description of motion to the causes of motion (forces and torques) and then to the action of forces and torques. The basic mechanical units are those of

MASS LENGTH and TIME

All mechanical quantities can be expressed in terms of these three quantities.

Physics is a discipline where experimental observations are recorded in numbers , the data, and these numbers are fitted with mathematical theories that describe the data and have predictive ability for new setups and observations.

The connection between data and theory is done with the laws of mechanics, in this case, and by the definition of the units in which the numbers are recorded.

Mass is an intrinsic property of the objects under observation. Entered into the equations its value is crucial in mapping the mathematics to observations.

To get the mass of an object one has to use the mathematics and the laws that govern the observations. It is the validation of the theory of mechanics by inummerable experiments and observations that dictates this.

The weight of an object is the force of gravity on the object and may be defined as the mass times the acceleration of gravity, w = mg.

Therefore a simple scale can tell you that the weight of two objects is equal: one can then see from the formula that as the acceleration of gravity is the same on the two parts of the scale, the masses will be equal. This does not give the value of the mass. Just the equality with another mass.

The mass of an object can be found since ancient times using Archimedes principle..

One should be careful to have clear that the weight and the mass are two different quantities , the weight can be derived from the mass, and can change depending on the gravitational field,(w=mg) , whereas the mass is invariant by construction of the theory.

weight on moon

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  • $\begingroup$ Gravitational mass can be determined as such but what about inertial mass ? $\endgroup$ Jan 5 '16 at 7:56
  • $\begingroup$ One can expand into electrodynamics. the e/m experiment gives the inertial mass of the electron once e is known from other experimental data.( again the problem of units) en.wikipedia.org/wiki/Mass-to-charge_ratio . imo one assumes that gravitational mass and inertial mass are the same , until experimental evidence arises that there is a difference ( the same as assuming a mass for an object after all) $\endgroup$
    – anna v
    Jan 5 '16 at 8:43
  • $\begingroup$ Now you made the kids who will read this mistake mass for weight for the rest of their lives. WHY???? Every halfway competent physics teacher in the world tries to make them get rid of this falsehood and you are talking it up here? Seriously? Even the OP was much farther along in his thinking than the weight experiment. Can I please, please, ask you to rewrite this? Please? $\endgroup$
    – CuriousOne
    Jan 5 '16 at 9:00
  • $\begingroup$ @CuriousOne nit picking, but I expanded $\endgroup$
    – anna v
    Jan 5 '16 at 10:09
  • $\begingroup$ Not nit picking, just reminding you of the plight of the physics teacher. Kids have an uncanny tendency to pick easy, false explanations over correct, complicated ones. My own physics teacher was adamant that the worst thing one could do was to teach forces with a force gauge, because inevitably kids would come to believe that the definition of force was given by the elongation of a spring rather than the acceleration of a mass. The same is true for the conceptual difference between mass and weight. If you don't believe me... ask a working physics teacher. $\endgroup$
    – CuriousOne
    Jan 5 '16 at 18:24
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Mass is one of the most core and complicated concepts in dynamics.

It isn't complicated. There's some ambiguities, in that there's inertial mass and active gravitational mass and relativistic mass and more. But nowadays when we say mass without qualification we mean rest mass. And that isn't complicated.

I have tried many books but I still don't have a good idea of how the mass of any object is determined relative to another.

The mass of a body is its resistance to change-in-motion. That's all it is. If we ignore friction, you can push a tricycle easily enough. A car is more difficult, because it has a greater mass. It exhibits a greater resistance to change in motion.

In The Science of Mechanics by Mach he says that two bodies isolated from all others are said to have same mass if they impart equal and opposite accelerations to each other. I think this assumes the third law. Also it is not that easy to measure their accelerations.

That's talking about active gravitational mass, which is really a measure of energy. A massless photon has a non-zero active gravitational mass. And it's a wave, not a body. An electron however is a body, and it's easy to measure its acceleration in response to a known force. That's how electron mass was measured.

Do we have any better and more useful definition of mass?

See above, and take a look at Einstein's E=mc² paper where he said "body" and "electron" on the same line. He said "the mass of a body is a measure of its energy-content". When you trap a massless photon in a mirror box, it increases the mass of that system. See Light is Heavy by van der Mark and (not the Nobel) 't Hooft. You know about pair production and electron diffraction and electron spin and the wave nature of matter. The electron is like the photon in the box, minus the box. Like it's in a box of its making. And electron-positron annihilation is like opening one box with another, whereupon each is a radiating body that loses mass. All of it.

Think of photon momentum as resistance to change-in-motion for a wave propagating on a linear path, think of electron mass as resistance to change-in-motion for a wave in a closed path, then think of your car as being made of electrons and similar things, because we can diffract protons and neutrons too. It isn't rocket science. It's simple.

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