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2016-01-14 16:14 GMT-03:00 John Denker <jsd@av8n.com>:
In the context of:
https://www.av8n.com/physics/thermo/state-func.html#sec-internal-energy
On 01/14/2016 08:23 AM, Herbert Schulz wrote:
Yes, imagine a gas instead of the spring. As the gas expands it
pushes the piston and does work.
I say it does /not/ do work. No work is done by (or on) the blue
system. There is no F·dx: At the place where there is a nonzero
force, there is no displacement. At places where there is a
nonzero displacement, there is no force.
the system is not in equilibrium, so you can not speak of internal energy
the mass center was not moving an then it was moving, so the center of
mass had a force acting on it
the system have kinetic energy coming from the work of that force
so you have some "not internal energy" transformed in kinetic energy.
the "not internal energy" is reflected in the expansion of the spring
the system is acelerated, so is its mass center is not a good reference
system
the system is expanding, so is not a rigid body.