Re: [Phys-L] ?conservation of _internal_ energy

[Diego Saravia <dsa@unsa.edu.ar>]

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.