Mass defines energy which defines mass

[guss]

Energy (a joule) is defined as

So, the definition of energy involves kilograms. However, because of E = mc^2, mass is another way of writing energy, or the same thing as energy.

It seems to me like this should bring up some sort of contradiction. If mass and energy go back and forth defining each other, isn't nothing really defined in the end? Like some sort of endless loop?

Just spilling out thoughts here. Anyone have any ideas?

 

[Drakkith]

E=MC^2 does not "define" mass or energy. It simply let's you convert one to the other in the right circumstances.

 

[guss]

E=MC^2 does not "define" mass or energy. It simply let's you convert one to the other in the right circumstances.

It says 1 kg = 9×10¹⁶ kg*m²/s², or that a kg is proportional to kg*m²/s². Not sure if that's the same point I made before though, or if this point I'm making is stupid.

 

[Drakkith]

What says that?

 

[guss]

Nuclear power plants. If I were to take a kilogram of matter, and convert it completely into energy, I would get that amount, would I not?

 

[Doc Al]

It says 1 kg = 9×10¹⁶ kg*m²/s², or that a kg is proportional to kg*m²/s².

No, E = mc² says that a 1 kg mass will have a rest energy of 9×10¹⁶ kg*m²/s² = 9×10¹⁶ Joules.

 

[guss]

No, E = mc² says that a 1 kg mass will have a rest energy of 9×10¹⁶ kg*m²/s² = 9×10¹⁶ Joules.

So 1 kg at rest has 9×10¹⁶ kg*m²/s² -- I'm not sure if that applies to my original question.

 

[Drakkith]

See here: http://en.wikipedia.org/wiki/Joule

Energy is also defined as: It is equal to the energy expended (or work done) in applying a force of one Newton through a distance of one metre (1 Newton metre or N·m), or in passing an electric current of one ampere through a resistance of one ohm for one second.

Neither of those involve mass at all.

Also, realize that mass and energy are not the same thing. A block of iron has mass, but it does not necessarily have energy. Using E=MC^2 only gives you the amount of energy you would get IF you could convert all of that mass into energy.

 

[Drakkith]

Per the wiki article on energy: In physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation"[1]) is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems.[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

That is a definition of energy. E=MC^2 is not.

 

[Pengwuino]

I think you might be confusing what variables are vs. what units are. Units are systems used to be able to differentiate between different physical phenomena in a convenient fashion, but you can't look at physical relationships by looking at units.

For example, torque and energy have the same units, but that doesn't mean any equivalence can be made between the two simply by looking at the units involved.

 

[guss]

Neither of those involve mass at all.

It involves force, which is kg*m/s².

That said, I think I understand now. Thanks everyone.

 

[granpa]

in a universe where all particles had the same mass
force and acceleration would always be proportional.
the concept of mass would be redundant.

 

[Pengwuino]

in a universe where all particles had the same mass
force and acceleration would always be proportional.
the concept of mass would be redundant.

No it wouldn't. There is still the big matter of varying distances between particles. In fact, at the classical level, I don't think a single bit of complexity would be done away with if mass was only divisible to a certain, fundamental point.

 

[granpa]

huh?
fields would still exist and the force would vary with distance but acceleration would always be proportional to force.