E=mc2 involve mass and energy as relative values?

Einstein's E=mc2

Homework Equations

E=mc2,
mass dilation, relativistic energy.

The Attempt at a Solution

Does Einstein's E=mc2 involve mass and energy as relative values? or does the equation mc2=moc2+Ek ie: the same eqn just with mass dilation and relativistic energy involved...?

I'm doing a presentation and I want to say that the second eqn above is the relativistic one and that einstein's one is the general one, but I have conflicting sources. If einstein's E=mc2 is relative then why doesn't it include mass dilation???

Answers and Replies

i dont knw much but m is the mass defect, like m mass con be converted into mc2 energy

as in neuclear reactions ... the mass lost in the rxn , lets say m gives mc2 energy energy

I think Einstein just generalized it, basically he was saying no matter what, the amount of energy in an object is always gonna equal the mass times the speed of light and vice versa. So basically "if that object became complete energy it would have this much energy or something like that." The relativistic eqn is more like, this object, if its moving at this speed or Ek will have this much mass, and this much energy.

Right>?

well yes, and its speed of light square

as much as i have used it, i think this equation is given for rest masses

and is Ek the kinetic energy?

Pengwuino
Gold Member

If you're looking at it as $$E_{total} = \gamma m_0 c^2$$, you could identify $$m=\gamma m_0$$ as the relativistic mass. This would be what you have. A quick manipulation gives $$E_{total} = m_0c^2 + (\gamma - 1)m_0c^2$$ where the second term is the kinetic energy

Yes, thats the equation im referring to, just slightly simplified.
I think I'm understanding this now,one more question tho?

The relativistic version of the mass-energy equivalence is compared with momentum because the faster a mass goes, the more momentum it has, which causes its relative mass to increase with more energy????

How does this apply with relative energy?

which in my textbook is mc2=mo(c2)+Ek

Could someone just reply and say if these points makes sense for relativistic mass,

-The other measure of mass (relativistic mass) is dependent on the velocity of the observer.
-This relativistic mass is a product of the increase in momentum relative to an external frame of reference

tiny-tim
Science Advisor
Homework Helper
hi skoks!
Does Einstein's E=mc2 involve mass and energy as relative values? or does the equation mc2=moc2+Ek ie: the same eqn just with mass dilation and relativistic energy involved...?

either …

in e=mc² , rest-energy and rest-mass (ie energy and mass at zero speed) is usually intended, but the same formula applies to (relativistic) mass and energy at any speed, and even to matter with zero rest-mass (at the speed of light)

Oh ok,

SO the equations in my textbook that aren't E=mc2 are just seperate eqns for the subtopics of relativistic momentum and energy, and they have nothing to do with E=mc2... ??

I havean eqn that combines dilated mass and momentum and an eqn that combimes E=mc2 dilated mass, and kinetic energy.

tiny-tim
Science Advisor
Homework Helper
SO the equations in my textbook that aren't E=mc2 are just seperate eqns for the subtopics of relativistic momentum and energy, and they have nothing to do with E=mc2... ??

it's very unusual for an exam question to involve e = mc² …

the question would have to involve the destruction of matter (there was a thread last week on a hypothetical rocket engine that completely converted fuel into photons! ) …

so most relativity equations have nothing to do with it

I understand!
Thanks a lot for the quick replies everyone!
I love this site.