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On 8/5/24 2:15 AM, Matthew Heaney via Phys-l wrote:
The force is equal to the (negative of the) derivative of the
potential energy.
Yes. Objects experience a force in the downhill direction.
Systems tend towards a state of lowest energy.
Nope. It says that in every 3rd-grade science book, but it's
not true. It even appears in science teaching standards so
that people are /required/ to teach it, but that doesn't make
it true.
If you change it to say lowest /potential/ energy it's still
not true.
In fact, force is not motion.
A planet in a circular orbit moves perpendicular to the force,
at constant potential energy and constant total energy.
A planet in an elliptical orbit spends half its time moving
downhill and half its time moving uphill.
If you place a drop of water at the bottom of a large container
of dry helium, the water molecules will evaporate and move up
into the bulk ... even though each water molecule is heavier
than each helium atom.
In fact it is *entropy* not energy that provides the answer to
all questions about spontaneity and irreversibility.
Third-graders don't know much about entropy, but as the saying
goes, just because the right answer is complicated doesn't make
it OK to teach the wrong answer.
In *some* cases a decrease in energy can be used as a proxy to
indicate an increase in entropy. In *some* other cases the Gibbs
free enthalpy can be used as a proxy. In general, though, using
entropy directly is the right answer.
The next level of detail can be found here:
https://www.av8n.com/physics/thermo/spontaneous.html
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