more on Brian Greene: That Famous Equation and You

[David Farber <dave () farber net>]

Begin forwarded message:

From: Brian Sniffen <bts () alum mit edu>
Date: September 30, 2005 2:22:46 PM EDT
To: dave () farber net
Cc: brett () lariat org
Subject: Re: [IP] Brian Greene: That Famous Equation and You


> Alas, the above are common misconceptions that bear correcting. When
> one burns gasoline, one doesn't convert any of its mass into energy.
> One is merely allowing the chemical bonds in the gasoline (and the
> air used to burn it) to rearrange themselves so that the resulting  
> atoms
> and molecules have less chemical potential energy.

This certainly does involve a change in mass.  A mole of O2 has a
different mass from two moles of O.  You need to look at very large
amounts of matter to see this effect, but the chemical binding energy
is mass, nonetheless.

You can read about atomic and chemical binding energies a bit here:
http://www.einstein-online.info/en/spotlights/binding_energy/


> It's the equivalent of allowing an anvil to descend from the top of
> a building to the bottom, and using a rope attached to the anvil to
> turn a shaft or do other work as it falls. The anvil has precisely
> the same mass at the bottom as it did at the top; it merely has less
> gravitational potential energy because it is closer to the center of
> the Earth. A similar process occurs on a molecular level when a
> battery is discharged or gasoline is burned.

I *think*, but am less certain, that a dropping anvil also exhibits a
change in mass.  You'd need to drop an awfully big anvil an awfully
long way to be able to measure it, though.  That anvil's falling
deeper into a gravity well, so the potential energy has to go
*somewhere*.  It goes into rest mass.

-Brian

--
Brian Sniffen                                       bts () alum mit edu


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