Re: [Phys-l] Conservation of energy in nuclear reactions

["Robert Cohen" <Robert.Cohen@po-box.esu.edu>]

On Friday, May 01, 2009 11:54 AM, John Denker wrote:

> Part of the energy is mc^2.  We call this the _rest energy_.
> I suppose you could define potential energy to be equal to 
> the rest energy, but not everybody does.  Also there are 
> issues with the gauge-dependence of typical notions of 
> potential energy.

OK

> Let's be clear:  The equation E=mc^2 applies in the rest 
> frame of the particle.  This is Einstein's original meaning 
> of the equation, and also the meaning that has been 
> conventional among those who do relativity for a living, and 
> has been for several decades that I know of.  (I am aware 
> that from time to time there have been attempts to define 
> some notion of "relativistic mass", but this is a Bad 
> Idea./2/)  Mass (m) means rest mass, so it is redundant but 
> harmless to call it "rest" mass.  

Agreed.

> Bottom line:  Energy is conserved.  Period.  There are no 
> known exceptions.  Of course to make this work you need to 
> include all forms of energy, including the rest energy mc^2.

Still, the question is whether this form of energy is *in addition to*
the energies we considered previously (like the gravitational energy or
whatever) or *instead of*.  That is what I am confused about and how I
could simply explain this to non-scientists.

Consider an astronaut dropping a rock on the moon.  While the rock is
falling, how does the kinetic energy of the moon/rock/astronaut change?
How does the rest energy of the moon/rock/astronaut change?  Are there
any other forms of energy that are changing?

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University 
570.422.3428   rcohen@po-box.esu.edu    http://www.esu.edu/~bbq