Is the Law of Conservation of Mass Always Obeyed?

[Nick tringali]

Are the laws of conservation of mass always obeyed. in a nuclear reaction mass is converted to energy therefore mass isn't conserved. I understand that energy is always conserved, but I feel like mass isn't.

 

[dRic2]

I'm totally ignorant about nuclear reaction, but I know conservation of mass/energy in nuclear reaction follows the rule $E = mc^2$. If you lose some mass you have to gain an equal amount of energy and vice versa. (very brutally said)

 

[phinds]

Are the laws of conservation of mass always obeyed. in a nuclear reaction mass is converted to energy therefore mass isn't conserved. I understand that energy is always conserved, but I feel like mass isn't.

Since you just stated, correctly that in a nuclear reaction mass is converted to energy, why do you "feel" that mass isn't conserved rather than "know" that mass isn't conserved? What IS conserved in a nuclear reaction?

 

[phinds]

I'm totally ignorant about nuclear reaction, but I know conservation of mass/energy in nuclear reaction follows the rule $E = mc^2$. If you lose some mass you have to gain an equal amount of energy and vice versa. (very brutally said)

@dRic2, on PF we try to help people figure out how to get answers for themselves rather than just giving them the answers.

 

[Nguyen Son]

As I know, law of conservation of mass is not always obeyed. You can read more about it here https://en.wikipedia.org/wiki/Conservation_of_mass and https://en.wikipedia.org/wiki/Conservation_law. Hope it can help

 

[dRic2]

@dRic2, on PF we try to help people figure out how to get answers for themselves rather than just giving them the answers.

I'm very sorry, I thought the OP got a little confused but wasn't far from the explanation. I meant the answer as a hint to find other material online because, as I said, I don't know much more about it.

 

[phinds]

I'm very sorry, I thought the OP got a little confused but wasn't far from the explanation. I meant the answer as a hint to find other material online because, as I said, I don't know much more about it.

Sure. I understand that you were being helpful and I applaud that. I was just concerned that your hint was more like an answer. I was trying to get him to think about it. You are right that he basically already HAD the answer he just didn't seem to have thought it through.

 

[Nick tringali]

yes, so mass isn't conserved. Energy is, but then why is the "Law of conservation of mass" a thing.

 

[phinds]

yes, so mass isn't conserved. Energy is, but then why is the "Law of conservation of mass" a thing.

Google is your friend
https://en.wikipedia.org/wiki/Conservation_of_mass

 

[Nick tringali]

I'm looking for more of a definitive answer as to if the law of conservation is not obeyed all the time, then why does education make it seem that it is always obeyed and mass is always conserved. Thanks anyways.

 

[phinds]

I'm looking for more of a definitive answer as to if the law of conservation is not obeyed all the time, then why does education make it seem that it is always obeyed and mass is always conserved. Thanks anyways.

In education, the process is first you learn to crawl, then walk, then run. One does not start out teaching Quantum Mechanics and General Relativity.

 

[Nick tringali]

Okay so mass is conserved most of the time but it is not always conserved. However, energy is always conserved in every way shape and form.

 

[Nick tringali]

since we are on this topic, is there any time that energy isn't conserved.

 

[phinds]

since we are on this topic, is there any time that energy isn't conserved.

Yes. When it is converted to mass.

 

[Orodruin]

However, energy is always conserved in every way shape and form.

As with everything, conservation of energy has limits to its applications as well. However, those details are probably best left until A-level general relativity ... In the meantime, you can consider energy as conserved.

 

[Orodruin]

Yes. When it is converted to mass.

Well ... I do not believe this is a constructive way of looking at things. The natural interpretation is that mass is a form of energy and that non-conservation of mass is just conversion of mass into other forms of energy.

 

[nikkkom]

Are the laws of conservation of mass always obeyed.

Depends on what you call "mass".
For example, in system of units where speed of light is 1 the E=mc^2 becomes just E=m. IOW: "energy is mass".
OTOH, another common (maybe even most common now) use of term "mass" is that mass is the length of 4-momentum vector of Special Relativity: m^2 = E^2 + p^2.
With both above definitions, "mass" is conserved.

 

[nikkkom]

another common (maybe even most common now) use of term "mass" is that mass is the length of 4-momentum vector of Special Relativity: m^2 = E^2 + p^2.

Er... I meant m^2 = E^2 - p^2, of course.

 

[jartsa]

Let's all think about this scenario:

We have a nuclear car that accelerates from 0 to 100 m/s. This happens on a race track on earth.A: Does the energy of the car change? B: Does the mass of the car change?

My answer:
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.
.
.
.
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A: The change of the the energy of the car depends on the frame. B: The mass of the car decreases.

So mass of the car is not conserved, and neither is the energy of the car.

 

[nikkkom]

A: The change of the the energy of the car depends on the frame. B: The mass of the car decreases.

So mass of the car is not conserved, and neither is the energy of the car.

Incorrect. Energy, of course, is frame-dependent. This does not invalidate energy conservation.
Car's energy is likely not conserved since it's not a closed system - it pushes against the Earth, the radiation and heat is escaping the car, etc.

 

[jartsa]

Incorrect. Energy, of course, is frame-dependent. This does not invalidate energy conservation.
Car's energy is likely not conserved since it's not a closed system - it pushes against the Earth, the radiation and heat is escaping the car, etc.

The energy of the car is not conserved in all frames.

If we take into account the energy that the Earth gains or loses, then energy is conserved, in all frames.

And:

If we take into account the earth, then mass is conserved. We just have to use the relativistic mass addition formula to combine the mass of the car and the mass of the earth.

 

[ChrisVer]

The energy of the car is not conserved in all frames.

In which frame is the energy conservation violated?

 

[jartsa]

In which frame is the energy conservation violated?

Well the energy of the car changes in those frames in which the energy of the Earth changes - so that energy conservation is not violated.

 

[f todd baker]

The simple fact is that there is no such thing as conservation of mass. In a chemical reaction, burning something for example, the thermal energy after the burning is the result of the initial mass becoming smaller. However, chemistry is such a poor way to get energy that the mass changes are extraordinarily small, so early chemists would not possibly observe it and assumed that the mass in a chemical reaction did not change, an empirical observation. Lucky thing, though, because it would probably have taken a much longer time to understand chemistry without "conservation of mass".

 

[DrStupid]

The simple fact is that there is no such thing as conservation of mass.

This "fact" might be simple but it is wrong. The mass of an isolated system is constant and that's what conservation means.

In a chemical reaction, burning something for example, the thermal energy after the burning is the result of the initial mass becoming smaller.

Neither energy nor mass are changing during this process. They are both conserved and can't be converted into each other or into something else.