[nyc111]

Do we actually know how physicists define "mass" in this context? Because, in physics many words have technical meanings that only physicists can know. For instance, as a layman when I see the word "particle" I imagine a spherical thing with an extension in space. But a physicist would laugh at me because in physics a particle is not a particle, it can be a statistical bump in data, it can be a field, it can be a wave, anything but a spherical particle. But a physicist would call a wave a particle and see nothing wrong with it. The same goes for mass, what physicists call mass can be voltage for instance. So does anyone know what "mass" means in this context?

[cowboysauce]

> The same goes for mass, what physicists call mass can be voltage for instance.

You’re probably thinking of how a proton has a mass of 938 MeV/c^2. This is still a mass and not a voltage. 1 eV (electronvolt) is the amount of kinetic energy that an electron would have after being accelerated though an electric potential of one volt. By the mass-energy equivalence 1 eV is equivalent to a mass of ~1.783x10^-36 kg and a proton has a mass of ~1.673x10^−27 kg.

[nyc111]

> You’re probably thinking of how a proton has a mass of 938 MeV/c^2.

Yes, that’s what I was thinking. But it seems that there is a problem with the definition of the word “mass”. Clearly there are at least two definitions. First, the weight of an object. Here weight is measured and weight is called “mass”. There is no equivalence, same thing is called weight and mass. Weight and mass are synonyms. This mass has nothing to do with electricity and has nothing to do with motion.

The second definiton of mass is related to electricity and motion. It has no meaning outside electricity. In this case, they accelerate an electric current and measure its kinetic energy and call this kinetic energy “mass”. Again these words are synonyms. Why do physicists like these silly word plays so much, I have no idea.

[patrickkrusiec]

The weight of the object is not its mass. The weight is the force an object experiences due to a gravity. You have the same mass on the Earth and the Moon, but you weigh 1/6 of your weight on Earth on the Moon because of the gravitational force is 1/6th the strength of Earth's gravitational force.

The definition of mass is subtler, but you seem to confusing units with the definition of the concept. Units are necessary because you need a scale to measure physical properties. You can't measure a length and say that it's "ten". You needs units attached like feet or meters. An eV (electronvolt) is a unit of energy. Just like a kilogram (unit of mass) originally was defined as the mass of a 10cm x 10cm x 10cm cube of water at room temperature, the eV (unit of energy) is defined as the increase in kinetic energy of an electron (which has a fixed and known charge) accelerated across 1 volt. But neither the definition of the kilogram or the eV define the concepts of mass or energy, they just merely define units, which humans chose, used to measure mass or energy.

Now how does mass and energy relate to each other? Simply put, the Special Theory of Relativity, developed by Einstein in 1905, states that mass and energy are equivalent to each other. Now the word "equivalent" has a precise but complicated meaning that I will not explain here (if you do want to understand it, take a course in special relativity). This relation is defined quantitatively by E=mc^2 (Energy E equals mass m times the speed of light c squared). Let's first look at another relationship, distance = velocity * time. This equation can be be rewritten as distance/time = velocity. If we use meters to measure distance and seconds to measure time, we can "divide" the units and define the units of velocity as a meters per second or m/s. Same thing with E=mc^2. We can rewrite the mass m as m = E/c^2, and let the units of mass be eV/c^2. Using eV/c^2 or kilograms or whatever to measure mass has no effect on the definition of the concept mass itself (which you can think of as an intrinsic property of objects independent of units which affects their behavior in known ways).

Why do physicists make all of this so complicated? They don't. It is reality that is subtle and complex and hard to understand. Because the purpose of physics is to describe reality, it has to be subtle and complicated.

[BlueTemplar]

Particle and wave are just both incomplete ways to describe things - but for some phenomena combining both gives the best results : that is the very weird domain of quantum physics.

(Like if you're trying to predict what happens when speeds approach those of light, you have to make weird relativistic corrections stemming from the observed speed of light being the same for all observers, regardless of their relative speeds.)

If you're lucky to not be in one of those weird cases, objects as spherical things with a sharp boundary in space and using the normal composition of speeds work just fine.