Uncovering the Mystery of Mass: What is it?

[yhPscis]

I understand that mass isn't weight. When I look up "what is mass?", I come across all kinds of videos explaining the difference between mass and weight, but that is not what I am looking for. I'm trying to understand the concept of mass, fundamentally, because it doesn't really mean anything to me right now and makes it difficult for me to concretely grasp the many physical formulas and concepts that involve mass because of this.

Wiki defines it as "a property of a physical body which determines the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies", but all this says is what mass does, not what mass is.

To me this is like: If I ask "what is an atom?" and people respond "it's something that forms covalent bonds", I still don't know what this "something" is. If it's defined as "the smallest unit an object consists of", then I can mentally represent what the concept is about (because cutting things into smaller pieces is something I come across in my daily life); it becomes something more concrete. I'd like "mass" to be as clear in my mind, but it isn't.

Can you please explain it to me?
Do we even know what mass fundamentally is or do we only know it's there because we empirically observe its effects, but do not know what it fundamentally is?

Mass is sometimes defined as "the amount of matter" that something has, but even "matter" I don't really understand as a fundamental concept. Intuitively, I saw it as "the tangible elements that objects are made of", and objects are made of atoms, so that led me to try to understand it as the amount of atoms objects have, but that makes no sense because things are made out of billions and billions of atoms, so something that has a mass of 3 kg wouldn't exist. What does this "3" refer to if it isn't the amount of atoms? How can you quantify something that isn't fundamentally defined?

Maybe I sound a little dense, and I'm sorry for that, but I'm just not wrapping my head around this one. Thank you if you're willing to spell things out for me. =/

 

[ShayanJ]

We don't know what mass fundamentally is!
Some people may say its just another form of energy and they're right but then we should know what is energy which again we don't know.
I guess we need some undefined concepts and axioms to start any system including physics. Energy is one of those undefined concepts.

 

[Zoo]

Talking about mass. But only it's effect.
This is a difficult concept but maybe someone can figure this one out someday!
We don't know what the mind is. We don't know what the ego is. But we can see it's effect in this world.
A lot of things we only know exist because of the effects they have.
They only knew a blackstar existed because of the stars drawn to them.

 

[yhPscis]

Thank you for your answers!

We don't know what mass fundamentally is!
Some people may say its just another form of energy and they're right but then we should know what is energy which again we don't know.
I guess we need some undefined concepts and axioms to start any system including physics. Energy is one of those undefined concepts.

Talking about mass. But only it's effect.
This is a difficult concept but maybe someone can figure this one out someday!
We don't know what the mind is. We don't know what the ego is. But we can see it's effect in this world.
A lot of things we only know exist because of the effects they have.
They only knew a blackstar existed because of the stars drawn to them.

I now understand that mass is only defined by its effects, so I realize that my biggest problem is related to quantification. I don't understand how people came up with a system to quantify mass.

I tried to figure it out using the formula for mass, which is F= m . a and my issue was the following:

a: Acceleration as a concept is very straightforward. It's a measure of the distance in relation to the time in which it is traveled. In order to measure distance, we've taken an arbitrary amount of space and called it a meter and an arbitrary amount of time and called it seconds. Nothing to look for there, it's all arbitrary.

F: Same with force. In order to quantify it, we arbitrarily decided to equal 1 unit of force (Newton) with what is needed to make an object of 1 kilogram have an acceleration of 1 meter per second.

m: But when it comes to mass, I don't see what they used to quantify it (given that it's definitely not the amount of atoms contained in an object). You could say that it's that which causes the needed force to accelerate the object to be greater the bigger it is, but then there's a loophole in the definition (force is defined in terms of mass and mass is defined in terms of force). So what basis was used to standardize the measurement of the concept of mass?

 

[rtsswmdktbmhw]

So what basis was used to standardize the measurement of the concept of mass?

As with defining a meter or a second, mass was also arbitrarily chosen. The kilogram is a block of metal that sits somewhere in France (I think). That block of metal was defined to the 'the kilogram'.

 

[UltrafastPED]

According to Isaac Newton and other classical physicists - mass is the quantity of matter.

Matter in turn was stuff you can touch: dirt, water, air, rocks - but not light, heat, shadows.
Matter is stuff you can transform and manipulate. All forms of matter have mass, and you can show equivalent mass with a balance scale, or via weight on a calibrated spring scale.

Mass is a fundamental property of sub-atomic particles such as electrons, neutrons, protons. Each quantity of mass also contains energy (which you can obtain via nuclear reactions, for example) such that E=mc^2.

You may find this NIST article of interest:
http://physics.nist.gov/cuu/Constants/introduction.html

And proposed future standards, such as counting atoms:
http://en.wikipedia.org/wiki/Kilogram#Proposed_future_definitions

 

[jtbell]

The kilogram is a block of metal that sits somewhere in France (I think). That block of metal was defined to the 'the kilogram'.

http://www.bipm.org/en/scientific/mass/prototype.html

 

[Dale]

but all this says is what mass does, not what mass is.

Personally, I think this is a meaningless distinction. What does this even mean?

Let's suppose that there is a person and you ask "who is this person". I can say, "he is John". That is one way of identifying a person, his name. Now, you ask "but all this says is his name, not who he is"? So I start to describe John in terms of his properties "He is 5'10" tall, 36 years old, red hair, brown eyes, he is socially awkward, he likes football ...". Now you say "but all this says is what he is like, not who he is". So I can describe John in terms of his relationships "he is Mary's husband and Autumn's father and he runs the laundromat on 35th and State and he is part of ...".

If you still aren't satisfied then what else are you looking for? Beyond a name, a set of properties, and a set of relationships, what does it mean to you to know what something "fundamentally is"?

 

[ShayanJ]

Personally, I think this is a meaningless distinction. What does this even mean?

Let's suppose that there is a person and you ask "who is this person". I can say, "he is John". That is one way of identifying a person, his name. Now, you ask "but all this says is his name, not who he is"? So I start to describe John in terms of his properties "He is 5'10" tall, 36 years old, red hair, brown eyes, he is socially awkward, he likes football ...". Now you say "but all this says is what he is like, not who he is". So I can describe John in terms of his relationships "he is Mary's husband and Autumn's father and he runs the laundromat on 35th and State and he is part of ...".

If you still aren't satisfied then what else are you looking for? Beyond a name, a set of properties, and a set of relationships, what does it mean to you to know what something "fundamentally is"?

Of course...I think we were missing this. And this is exactly the reason that there should always exist axioms and undefined concepts because somewhere you run out of things to relate your concepts to and run out of properties to associate to concepts!

 

[dauto]

I have a similar problem. I don't know what's an apple. I've seen many web pages explaining that apples and bananas are not the same thing. Many pages also described an apple in terms of its properties. They mentioned that it is a sweet juicy fruit but all failed to tell me what it is. I understand that apples are a fruits but when I tried to understand what is a fruit I found that a fruit is defined in terms of its properties, origin, and biological function but all the pages I read failed to describe what a fruit really is. I still don't know what is an apple.

 

[SteamKing]

Thank you for your answers!

I now understand that mass is only defined by its effects, so I realize that my biggest problem is related to quantification. I don't understand how people came up with a system to quantify mass.

I tried to figure it out using the formula for mass, which is F= m . a and my issue was the following:

a: Acceleration as a concept is very straightforward. It's a measure of the distance in relation to the time in which it is traveled. In order to measure distance, we've taken an arbitrary amount of space and called it a meter and an arbitrary amount of time and called it seconds. Nothing to look for there, it's all arbitrary.

Acceleration is the time rate of change of the velocity, which in turn, is the time rate of change of position.

You will find that all measurement systems are arbitrary to some extent. The yard was based on the distance from someone's nose to the tip of his finger. The meter has been defined several times. Originally, it was supposed to be 1/10,000,000 of the distance along a quadrant of the Earth from the equator to the north pole. When it was found to be impractical to physically survey that quadrant, other equally arbitrary definitions were substituted.

The fact that something is arbitrary is not necessarily a disqualification to being useful. If enough people agree to observe the same set of arbitrary measurements, then we have a convention which can be applied consistently.

F: Same with force. In order to quantify it, we arbitrarily decided to equal 1 unit of force (Newton) with what is needed to make an object of 1 kilogram have an acceleration of 1 meter per second.

As a result of Newton's Second Law of Motion, force and mass were related by F = m a

m: But when it comes to mass, I don't see what they used to quantify it (given that it's definitely not the amount of atoms contained in an object). You could say that it's that which causes the needed force to accelerate the object to be greater the bigger it is, but then there's a loophole in the definition (force is defined in terms of mass and mass is defined in terms of force). So what basis was used to standardize the measurement of the concept of mass?

Originally, the gram was defined as the weight of 1 cc of pure water at the temperature of melting ice.

http://en.wikipedia.org/wiki/Gram

Later, other standards were developed, since water has a tendency to evaporate and it also changes density with temperature. A physical standard kilogram mass was fabricated and kept at Paris.

We can't measure mass directly, but we can measure the effect which gravity has on a standard mass. We can measure this effect on other bodies, and a comparison of the two measurements will determine the ratio of the mass of the given body to the standard.

 

[yhPscis]

Personally, I think this is a meaningless distinction. What does this even mean?

Let's suppose that there is a person and you ask "who is this person". I can say, "he is John". That is one way of identifying a person, his name. Now, you ask "but all this says is his name, not who he is"? So I start to describe John in terms of his properties "He is 5'10" tall, 36 years old, red hair, brown eyes, he is socially awkward, he likes football ...". Now you say "but all this says is what he is like, not who he is". So I can describe John in terms of his relationships "he is Mary's husband and Autumn's father and he runs the laundromat on 35th and State and he is part of ...".

If you still aren't satisfied then what else are you looking for? Beyond a name, a set of properties, and a set of relationships, what does it mean to you to know what something "fundamentally is"?

I have a similar problem. I don't know what's an apple. I've seen many web pages explaining that apples and bananas are not the same thing. Many pages also described an apple in terms of its properties. They mentioned that it is a sweet juicy fruit but all failed to tell me what it is. I understand that apples are a fruits but when I tried to understand what is a fruit I found that a fruit is defined in terms of its properties, origin, and biological function but all the pages I read failed to describe what a fruit really is. I still don't know what is an apple.

...dauto, I don't know if I'm supposed to take this as a mockery or a benevolent explanation of what you think is wrong with my way of thinking. If it's the former, know that it's very disturbing and I'd like you to not do that again in the future. It's counterproductive to try to embarrass people for doing what this forum is about - seeking help.

I'm not rejecting definitions in terms of any property, I gave the example of the atom to clarify what terms would help me understand. Then I further clarified that it was actually more a matter of how it's measured that was problematic to me.

If it's the latter then it's okay. Thank you all of you who made the effort to help me, I understand it a lot better already! :)

 

[Khashishi]

but then there's a loophole in the definition (force is defined in terms of mass and mass is defined in terms of force).

Actually, it's not all circular, because we have other equations which specify some properties of force and mass. One equation by itself tells you nothing, but when you have a whole set of equations which all fit together, you achieve a powerful consistent meaning.

$F=ma$
but we also know from Newton's third law:
$F_{12} = -F_{21}$
We can now combine the laws to say: If two objects are pushing off each other and accelerate at equal rates (in opposite directions), they have the same mass.

We also have the conservation of mass. By extension, if you add two masses together, you get a larger mass through addition. Now you can apply the same force to a different mass and calculate the acceleration.

Mass is just this quantity that appears in a lot of equations, and the fact that many equations use the same quantity makes it a powerful concept. I would not worry about any underlying meaning of mass beyond what is in the equations. Physics is just about making models that match our experiments. If you ask why too many times, you end up with philosophical nonsense.

 

[Dale]

Then I further clarified that it was actually more a matter of how it's measured that was problematic to me.

Ah, then I would recommend jtbell's post #7 link to the BIPM website.

 

[DrStupid]

but when you have a whole set of equations which all fit together, you achieve a powerful consistent meaning.
$F=ma$
but we also know from Newton's third law:
$F_{12} = -F_{21}$
We can now combine the laws to say: If two objects are pushing off each other and accelerate at equal rates (in opposite directions), they have the same mass.

It also works with the original second law (F=dp/dt). But than you additionally need the definition of momentum (p=m·v) and the transformation (Galilei or Lorentz) to get a full definition of mass (as used by Newton).

 

[dauto]

...dauto, I don't know if I'm supposed to take this as a mockery or a benevolent explanation of what you think is wrong with my way of thinking. If it's the former, know that it's very disturbing and I'd like you to not do that again in the future. It's counterproductive to try to embarrass people for doing what this forum is about - seeking help.

I'm not rejecting definitions in terms of any property, I gave the example of the atom to clarify what terms would help me understand. Then I further clarified that it was actually more a matter of how it's measured that was problematic to me.

If it's the latter then it's okay. Thank you all of you who made the effort to help me, I understand it a lot better already! :)

My comments were not intended as mockery. They are just my slightly dorky way of pointing out that there really isn't any difference between understanding what something is and understanding its properties.

 

[yhPscis]

Okay, thanks to you too then (the Internet makes it difficult to know how to take things, sorry!)

 

[Chestermiller]

I look at it differently. In my judgement, you were on the right track when you focused on F = ma. Suppose we could specify force independently of mass, by, say, measuring the tension in a spring. So force would not be just mass times acceleration, but something that stands on its own. Once we agree that this is possible, then we have a precise definition of mass. It is simply the proportionality constant between the net force that you apply to a body and the acceleration that the body experiences in response to the force.

Chet

 

[TSC]

There are at least three different definitions of mass.
(1) Quantity of matter
(2) Amount of inertia
(3) Gravitational charge.

 

[Andrew Mason]

Mass is sometimes defined as "the amount of matter" that something has, but even "matter" I don't really understand as a fundamental concept. Intuitively, I saw it as "the tangible elements that objects are made of", and objects are made of atoms, so that led me to try to understand it as the amount of atoms objects have, but that makes no sense because things are made out of billions and billions of atoms, so something that has a mass of 3 kg wouldn't exist. What does this "3" refer to if it isn't the amount of atoms? How can you quantify something that isn't fundamentally defined?

If one starts with the premise that:

1. the laws of motion are the same in all inertial reference frames, and
2. time and space are absolute,

the concept of mass as a quantity of matter naturally emerges.

We can define a unit of force as the physical phenomenon that causes a one unit change of velocity of a unit body in one unit of time. One unit of force applied to each of two identical unit bodies at the same time for the same duration would result in the same change of velocity. If that was not the case, then after the force ended (at the same time for both unit bodies), the bodies would define two different inertial frames of reference in which either: 1. the laws of motion were not the same, or 2. time and space were measured differently. In that case, one of the premises would be negated.

One unit of force on each body is a total of two units of force. Therefore, the force required to change the motion of two unit bodies in a given time period is double that required to make the same change of motion of a one unit body. So we conclude that the force is proportional to the number of unit bodies of matter for a given change in motion per unit time.

Newton conceived that matter must consist of fundamental units and that the mass of a macroscopic body depends on how many of these units it contains. He was basically right - very close. The units are nucleons - protons and neutrons. The mass of a macroscopic body is very close to being exactly proportional to the number of nucleons it contains. [There is a slight difference between the mass of a neutron and the mass of a proton+electron. Also the masses of different nuclei are not exactly proportional to the number of nucleons because of different binding energies, but the differences are very small.]

The definition of mass as "quantity of matter" begs the question: how does one define the mass of a particles smaller than a nucleon? For fundamental particles, such as electrons, neutrinos, positrons, quarks, anti-quarks etc., the definition of mass has to change, because they are not made up of nucleons. When defining something that is fundamental the best you can do is describe its qualities. For fundamental particles, we define their masses by the way they respond to forces as if they were macroscopic bodies, using m = F/a .

AM

 

[Andrew Mason]

Just as a follow-up on this interesting question, the issue - it seems to me - is not so much why acceleration is inversely proportional to mass for a given force (or why force is proportional to mass for a given acceleration). That is easy enough to explain with mass defined as the quantity of nucleons and using the kind of argument above.

Rather - it seems to me - the question is why some particles can define inertial reference frames (e.g. various combinations nucleons and electrons) and some can't (photons). Once you have a particle that defines an inertial reference frame, the concept of mass necessarily arises.

AM

 

[MathematicalPhysicist]

There are at least three different definitions of mass.
(1) Quantity of matter
(2) Amount of inertia
(3) Gravitational charge.

You might as well ask what is "charge", what is "inertia".

Or just do as I do, take it as a basic constituent of physics, like time, space, charge, etc.

Some things aren't definable you just take them as granted like:

 

[sophiecentaur]

My comments were not intended as mockery. They are just my slightly dorky way of pointing out that there really isn't any difference between understanding what something is and understanding its properties.

It doesn't sound dorky to me. 'Things' are only defined by their properties - in one way or another. One can feel familiar enough with some things to be tempted to think they are fundamental but they are only appreciated by virtue of their context with the rest of things. You see, feel, hear, taste some things and you can sometimes measure them - in terms of other things.

 

[ch@rlatan]

Hey yhPscis,

Mass is an elusive property. When you consider the masses of the individual quarks that constitute the proton or neutron, for example, and compare that with the mass of these composite particles, it becomes clear that there is something other than 'particles' that constitute mass. The difference can only really be resolved in the efficacy of the bonds that they make with each other. This is true at the chemical level as well as the quark and is characterised as the 'mass defect' or 'binding energy' of the particles involved. Remembering that mass is the same as energy through the principle of mass/energy equivalence.
Perhaps large binding energies are related to field inefficiencies (contorted field) of the binding particles but until New Physics from a Newton MarkII develops a model of particle structure, negating the need for the probabilistic nature of quantum mechanics, we just don't know.

ch@rlatan

 

[lendav_rott]

Don't know what mass is - energy, I suppose. If you stand on a bathroom scale it gives you a reading - is that your mass? (excluding the wiseassery like the clothes you are wearing and whatnot) Suppose you get to use the same scale on a much larger and denser planet - will you get the same reading? Can't say exactly, what mass is.

The only explanation that doesn't make my head hurt is that mass is energy.

 

[rtsswmdktbmhw]

If you stand on a bathroom scale it gives you a reading - is that your mass? (excluding the wiseassery like the clothes you are wearing and whatnot) Suppose you get to use the same scale on a much larger and denser planet - will you get the same reading? Can't say exactly, what mass is.

Bathroom scales are calibrated to Earth's gravitational field and does indeed show mass - on Earth. If you take it onto another planet it will no longer be correctly calibrated and won't show the same value until it is calibrated to the new planet's field strength. (Sorry if I used incorrect terminology.)

 

[sophiecentaur]

Bathroom scales are calibrated to Earth's gravitational field and does indeed show mass - on Earth. If you take it onto another planet it will no longer be correctly calibrated and won't show the same value until it is calibrated to the new planet's field strength. (Sorry if I used incorrect terminology.)

They measure the weight (the downwards force), which will correspond to a certain mass on Earth. A 'Balance' compared the test article with 'known' masses and will work anywhere where there is a gravitational field (as long as the field is uniform).

 

[TumblingDice]

A 'Balance' compared the test article with 'known' masses and will work anywhere where there is a gravitational field (as long as the field is uniform).

To add to sophiecentaurs "spot on" reply: A common example is a balance scale like the one you step onto at the doctor's office while the nurse slides the weights. Your mass is calculated against the known masses of the sliding weights.

 

[Andrew Mason]

The only explanation that doesn't make my head hurt is that mass is energy.

Which does not really explain mass because: energy is defined as the ability to do work, and work is defined in terms of mass (the dimensions of energy are mass x distance^2 x sec^-2).

AM

 

[bahamagreen]

If one thinks of mass as the quantity of matter...

Is this thinking that the fundamental substance of matter is of one kind, with an invarient density, so that different "quantities" of this matter would be distinguished strictly as different volumes of this constant density matter?

Or is this thinking of mass as a magnitude of inertial reaction to acceleration... the mass possibly comprised of whatever variable volumes of variable densities of matter required to provide the reaction to force?

The later does not really capture the sense of matter being fundamental if there are different densities of it.

The former is totally confounded by modern theory, but I'm wondering what the classical thinking was, here; Newton, for example... did he infer that fundamental matter was of one kind?

 

[Drakkith]

If one thinks of mass as the quantity of matter...

That leads to problems. For example, 2 protons and 2 neutrons unbound in free space have MORE mass than when they are bound together in an alpha particle. Same number of particles, less mass.

 

[sophiecentaur]

Which does not really explain mass because: energy is defined as the ability to do work, and work is defined in terms of mass (the dimensions of energy are mass x distance^2 x sec^-2).

AM

But no quantity is 'explainable' in isolation. All that you can do is show how a certain set of relationships have some common factor and you then give that factor a name and call it a quantity. Science isn't really a set of things that have relationships; it's more the other way round, in as far as we observe phenomena and manufacture 'quantities' in our heads to explain the structure. Dimensional analysis it a simple way to approach the problem and gives us (or suggest to us) these quantities.
The temptation can be to approach Science as if it's based on a 'God says' / Leggo parts basis - looking for a structure on those terms. I really don't think there's much joy in that direction. It's the same as the Feynman objection to the 'why' question.

 

[craigi]

There is something very weird about mass.

We have the inertial mass and the gravitational mass. For a long time they were considered to be the same thing, but until Einstein there was no principle that showed their equivalence. I can think of no other property of matter that has such a deep dual purpose. It would seem reasonable to expect a fundamental reason that they are the same thing. This might offer the answer:

http://arxiv.org/abs/1001.0785

 

[DrStupid]

We have the inertial mass and the gravitational mass. For a long time they were considered to be the same thing

They are not the same thing and in GR there is no gravitational mass at all. Newton considered inertial mass and gravitational mass to be proportional (not identical) because it has been shown experimental:

"But I understand this quantity everywhere in what follows under the name of the body or of the mass. That becomes known through the weight of any body: for the proportion to the weight is to be found through experiments with the most accurate of pendulums set up, as will be shown later."
[Philosophiae Naturalis Principia Mathematica]

If you try to use gravitational mass in GR (even tough it makes no sense) it will appear to be equal to inertial mass for bodies at rest but not at very high velocities (see http://home.comcast.net/~peter.m.brown/ref/mass_articles/Olson_Guarino_1985.pdf )

 

[Andrew Mason]

But no quantity is 'explainable' in isolation. All that you can do is show how a certain set of relationships have some common factor and you then give that factor a name and call it a quantity. Science isn't really a set of things that have relationships; it's more the other way round, in as far as we observe phenomena and manufacture 'quantities' in our heads to explain the structure. Dimensional analysis it a simple way to approach the problem and gives us (or suggest to us) these quantities.
The temptation can be to approach Science as if it's based on a 'God says' / Leggo parts basis - looking for a structure on those terms. I really don't think there's much joy in that direction. It's the same as the Feynman objection to the 'why' question.

The "explanation" was sought by lendav_rott. My point was that "mass is energy" doesn't explain mass in terms of other concepts, since energy is defined in terms of mass.

We can explain distance and time in terms of concepts that we can readily perceive: space and changes in positions of things.

In a way, mass can be understood in terms of distance and time. Mass of a unit body of matter is perceived by the magnitude of its change in motion due to a specified interaction. This is the change per unit time of its position as measured in its pre-interaction reference frame.

AM

 

[sophiecentaur]

The "explanation" was sought by lendav_rott. My point was that "mass is energy" doesn't explain mass in terms of other concepts, since energy is defined in terms of mass.

We can explain distance and time in terms of concepts that we can readily perceive: space and changes in positions of things.

In a way, mass can be understood in terms of distance and time. Mass of a unit body of matter is perceived by the magnitude of its change in motion due to a specified interaction. This is the change per unit time of its position as measured in its pre-interaction reference frame.

AM

I would put it, rather, that we can show a relationship between - rather than "explain": that's one stage back in the process (Further from the answer to the 'why' question, if you like.
(I realized that you were not seeking an "explanation". I was endorsing your post. It's sometimes difficult not to appear to be disagreeing.)

 

[Aerion]

I've thought a lot about this, and I do think it is important to consider that our ideas about matter/mass/gravity are significantly influenced by the way our brain interprets data from the world. We think of matter as concrete, we associate matter with things. It is deceptively easy to think of particles as tiny balls of "stuff", and I think it is here that the confusion arrises. It seems more helpful to me to just think of particles as fields, with a central point where the field is most powerful, but nothing differentiating that center from the rest. There is no special part that could be "touched".

 

[lightarrow]

My point was that "mass is energy" doesn't explain mass in terms of other concepts, since energy is defined in terms of mass.

What do you mean? A photon's energy is defined as E = hf, the electrostatic field energy U in void is defined as U = eps_0 integral E^2dv where E = electric field, etc.

--
lightarrow

 

[Andrew Mason]

What do you mean? A photon's energy is defined as E = hf, the electrostatic field energy U in void is defined as U = eps_0 integral E^2dv where E = electric field, etc.

--
lightarrow

E=hf is not a definition. E relates to the ability of the photon to perform work on matter. Its ability to do work on matter is proportional to its frequency.

Energy has units of mass x distance^2/time^2 and it is defined as the ability to do work. Energy is not only defined in relation to mass - it only has meaning in relation to matter/mass.

AM

 

[lightarrow]

E=hf is not a definition. E relates to the ability of the photon to perform work on matter. Its ability to do work on matter is proportional to its frequency.

Energy has units of mass x distance^2/time^2 and it is defined as the ability to do work. Energy is not only defined in relation to mass - it only has meaning in relation to matter/mass.

AM

Sorry but it's your definition of energy that is not a definition at all. "Ability to do work"? Let's not joke...

--
lightarrow

 

[Andrew Mason]

Sorry but it's your definition of energy that is not a definition at all. "Ability to do work"? Let's not joke...

--
lightarrow

It is not exactly my definition. Work is defined in terms of force (mass x acceleration) and distance. If you have a better one, you should enlighten us.

AM

 

[lightarrow]

It is not exactly my definition. Work is defined in terms of force (mass x acceleration) and distance. If you have a better one, you should enlighten us.
AM

No, I don't mean to enlighten anyone, because it's impossible to give a definition of energy in a few words and which is comprehensive of all the forms of energy known.
About force, usually it's defined as dp/dt but you can define it with springs, as it were defined in the past. The concept of mass came after the concept of force, not before.

Having the concept of force as what is given from a compressed spring, you can define work as dW = Fdx, without need of knowing mass.

Of course nowadays we prefer to define F as dp/dt, because of convenience reasons.

--
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[Andrew Mason]

No, I don't mean to enlighten anyone, because it's impossible to give a definition of energy in a few words and which is comprehensive of all the forms of energy known.
About force, usually it's defined as dp/dt but you can define it with springs, as it were defined in the past. The concept of mass came after the concept of force, not before.

Having the concept of force as what is given from a compressed spring, you can define work as dW = Fdx, without need of knowing mass.

Of course nowadays we prefer to define F as dp/dt, because of convenience reasons.

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Actually F=dp/dt came from Newton but the concept of energy was not used until the early 19th Century.

While one can conceive of "force" independently of mass and acceleration (ie a compressed spring), the only way to experience a force is by its application to matter. And the only way to measure a force is by its interaction with matter e.g. the resulting acceleration of a mass. If you take a compressed spring and simply remove the constraint so that it expands freely (ie. without being in contact with a mass), it does no mechanical work at all.

AM

 

[lightarrow]

Actually F=dp/dt came from Newton but the concept of energy was not used until the early 19th Century.

Ok, but you wrote that energy is "the ability to do work" and then you wrote that "work is defined in terms of force (mass x acceleration) and distance" so having force you authomatically have energy, according to your definition.

While one can conceive of "force" independently of mass and acceleration (ie a compressed spring), the only way to experience a force is by its application to matter. And the only way to measure a force is by its interaction with matter e.g. the resulting acceleration of a mass.

I don't agree with that. If a laser beam is reflected off something (which has or not mass), its own momentum changes in a finite time, so we can say it experiences an average force that we could define as F = \Delta p/ \Delta t and we know how to compute/measure the beam's momentum p from its power and lenght, for example.

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[Andrew Mason]

Ok, but you wrote that energy is "the ability to do work" and then you wrote that "work is defined in terms of force (mass x acceleration) and distance" so having force you automatically have energy, according to your definition.

No. You need to apply the force through a displacement. One does not require energy to maintain a static force. When you drive a screw to pull two boards tightly together, that binding force will last for a very long time. During that time, no energy is required to keep the boards tightly together.

I don't agree with that. If a laser beam is reflected off something (which has or not mass), its own momentum changes in a finite time, so we can say it experiences an average force that we could define as F = \Delta p/ \Delta t and we know how to compute/measure the beam's momentum p from its power and lenght, for example.

The concept of force over time (impulse) does not apply to a photon.

The only way we can detect a photon is when it interacts with matter. What it is doing between the time it is emitted and the time it is received is unknowable.

Of course, a photon takes momentum from the matter that emits and delivers it to the matter that receives it.
The concept that a photon has momentum while it is a photon is a useful mathematical tool that helps us to visualize the transfer of momentum from one body to another and to preserve the principle of conservation of momentum.

AM

 

[lightarrow]

The concept of force over time (impulse) does not apply to a photon. The only way we can detect a photon is when it interacts with matter.

I have not mentioned photons at all.

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[Andrew Mason]

I have not mentioned photons at all.

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I assumed you were aware that a laser beam consists of a stream of (identical) photons.

AM

 

[lightarrow]

I assumed you were aware that a laser beam consists of a stream of (identical) photons.

AM

But I think this is irrelevant: I used (properly, improperly, don't know) simply the definition of force as Δp/Δt and the classical description of electromagnetic radiation.

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[lightarrow]

No. You need to apply the force through a displacement. One does not require energy to maintain a static force.

Maybe I didn't express myself clearly enough. Obviously there is the need of a displacement. But to define work we need force + displacement; since we already know how to define/make/measure a displacement, what is missing is the concept "force" and we were discussing about it for this reason. Having defined force we have everything we need to define work. This is what I intended.
Regards.

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[Andrew Mason]

Maybe I didn't express myself clearly enough. Obviously there is the need of a displacement. But to define work we need force + displacement; since we already know how to define/make/measure a displacement, what is missing is the concept "force" and we were discussing about it for this reason. Having defined force we have everything we need to define work. This is what I intended.
Regards.

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The issue is: how does one define a concept of force other than in relation to mass?
Force is implicitly defined in Newton's first law as something that causes a change in the motion of body i.e. a body of matter.The second law quantifies the force by relating the magnitude and direction of the force to the magnitude and direction of the acceleration of a mass.

You can suggest that it be standardized in relation to a standard spring but that only has meaning if there is mass involved. For example, I could say that a unit of force is the push provided by a standard spring with standard spring constant k compressed a by a standard displacement of x metres. But that unit of force can only exist if there is mass to push against. If it does not push on a mass, there is no force.

Similarly, if I punch with my fist in the air, I am not exerting force on anything (except a tiny force on the air). If I punch you, I may apply a force. If I punch a piece of paper with the same strength, I apply a much smaller force to the paper.

AM