Rest Mass Definition: What is E=MC2?

In summary: To say that "rest mass is energy that is stationary" is not accurate because, as the other poster pointed out, "stationary" is a relative term and depends on the reference frame. So it would be more accurate to say that rest mass is a form of energy that is invariant (unchanging) in all reference frames.In summary, rest mass is the mass of an object measured in a frame of reference that is stationary with respect to the object. This mass is equivalent to energy and remains proportional to it. The famous equation E=mc2 represents the relationship between mass and energy, but they are not the same thing. Rest mass is a complicated concept with multiple aspects and is not accurately described as "energy that is stationary
  • #1
avito009
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4
I was remembering the equation E=MC2. So as per me Rest Mass is nothing but Energy that is stationary. Am I right?
 
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  • #2
The "rest mass" is the mass of an object measured in a frame of reference that is stationary with respect to the object.
The mass may be "moving around" in some other reference frame, but it still has rest mass - therefore rest mass can be energy that is moving around.

Note: ##E=mc^2## just means that mass is a form of energy.
 
  • #3
Simon Bridge said:
Note: E=mc2E=mc^2 just means that mass is a form of energy.
Rest mass and rest energy are equivalent and remain proportional to one another. I am not talking about object that is moving here.

Now E=MC2. Energy and mass are equivalent and the "C2" is the conversion factor because the mass has to be converted into units of Energy. Mass is measured in Kilograms and Energy is measured in Joules. So to convert Kilograms to Joules "C2" would be used.
 
  • #4
Rest mass and rest energy are equivalent and remain proportional to one another. I am not talking about object that is moving here.
... this is correct. What I want you to realize is that "rest mass" does not refer only to "energy that is stationary". In the context of Einstein's relativity there is no absolute motion, so there is no absolute for "stationary" either. The word "stationary" must be used with a reference frame. Stationary with respect to what? Moving with respect to what? So when you said:
Rest Mass is nothing but Energy that is stationary.
... you forgot to say what it is stationary with respect to.
The statement is correct if you mean "stationary with respect to itself"... suddenly does not sound so profound does it :)

Now E=MC2. Energy and mass are equivalent and the "C2" is the conversion factor because the mass has to be converted into units of Energy. Mass is measured in Kilograms and Energy is measured in Joules. So to convert Kilograms to Joules "C2" would be used.
... the kg and the J are the SI units for mass and energy respectively. These are not the only units. Speed is also measured in miles-per-hour, mass in pounds, and energy in ergs, for example.

When you do relativity a lot you will start using "natural" units... these are units chosen so that c=1 - so the equation becomes E=M.
http://en.wikipedia.org/wiki/Natural_units

[edit] The ##c^2## is not there just to convert the units - if unit conversion were the only reason, then any old speed will do. For instance, the mass multiplied by the escape velocity from the surface of the Earth would have units of energy...

Niggle: usually the rest mass m and the invariant speed c are represented in lower-case letters.
The full mass-energy relation is ##E=\gamma mc^2##
 
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  • #5
avito009 said:
I was remembering the equation E=MC2. So as per me Rest Mass is nothing but Energy that is stationary. Am I right?
Rest mass is also known as invariant mass and is given by ##m^2c^2=E^2/c^2-p^2##. So if momentum is 0 then that reduces to the famous ##E=mc^2##.

Note that it is a mistake to say "energy is mass" since they differ when momentum is not 0 even in units where c=1 (mass is invariant and energy is not), but when (as you said) the energy is stationary then it is numerically equal to mass in units where c=1.
 
  • #6
avito009 said:
I was remembering the equation E=MC2. So as per me Rest Mass is nothing but Energy that is stationary. Am I right?

Remember that mathematical equations are not definitions. For example, the equation for electric current is I=V/R, or current equals voltage divided by resistance. But this doesn't mean that current is voltage or resistance, or that voltage or resistance is current. Each one has its own definition and all three are related, but they are not the same thing.

Similarly, mass and energy are related by the equation E=MC2, but that doesn't necessarily mean that energy and mass are the same thing. (Of course it doesn't mean that they aren't the same thing either) If I remember correctly, Einstein's original thought process regarding energy and mass was that if a quantity of energy is added or removed from a system, a corresponding amount of mass is added or removed as well, with the amount of mass and energy related by Einstein's equation.

Mass is a complicated concept that has multiple aspects. Wiki lists them here: http://en.wikipedia.org/wiki/Mass#Definitions_of_mass
 

1. What does E=MC2 mean?

The equation E=MC2 is known as the mass-energy equivalence formula, where E represents energy, M represents mass, and C represents the speed of light in a vacuum. It states that energy and mass are interchangeable and are related by the speed of light squared.

2. What is rest mass?

Rest mass is the mass of an object when it is not moving. It is also referred to as invariant mass or proper mass. It is a fundamental property of particles that does not change with velocity or location.

3. How is rest mass related to E=MC2?

E=MC2 shows the relationship between energy and mass, stating that energy is equal to the mass of an object multiplied by the speed of light squared. This equation applies to both rest mass (when an object is not moving) and relativistic mass (when an object is moving at high speeds).

4. Why is E=MC2 important?

E=MC2 is important because it explains the relationship between mass and energy, which is a fundamental concept in physics. It has also been used in various technological advancements, such as nuclear energy and nuclear weapons.

5. Can E=MC2 be used to convert mass into energy?

Yes, E=MC2 can be used to convert mass into energy. This is the basis of nuclear reactions, where a small amount of mass is converted into a large amount of energy. However, this conversion is only significant for very large masses, such as those found in atomic nuclei.

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