Actually, that is not my problem.
As repeatedly stated, "no work" is not the same as "no forces".
Clarify this statement please iCare .
Sure, no problem. I have already done so in response to one of your previous posts:
Joules law of free expansion of gas into a vacuum . No work is done therefore no force therefore no reaction therefore no acceleration .
Again, as explained before:
- No work is not the same as no force applied. If you hold a weight stationary with your arm extended (for fun, imagine a mug of beer), no work is done, as the object doesn't move; however upward and downward forces are applied - in this case they cancel each other out.
- Joule's law of free expansion does not apply, because it describes a completely different process.
I also referred to it in one of my recent posts:
"No work done" (comparing initial state and end state) does not mean "no force"; it means - in this case - same temperature at the beginning and at the end of the experiment.
Free expansion works, because at a certain temperature gas molecules have a specific kinetic energy that has them moving around randomly.
To expand on this:
Depending on the temperature of a gas it's molecules will have a certain kinetic energy that has them "bouncing around randomly".
They bounce against (and reflect) each other, but they also bounce against the walls of a container that is enclosing them => manifesting as "pressure".
Heat the gas => more kinetic energy => more pressure (for cooling it's the other way round).
Enlarge the size of the container => less hits on the enclosure => less pressure (for reducing the size of the container it's the other way round).
Now, here it gets a bit tricky, as compressing/decompressing gas can actually change its temperature, but let's ignore that and look at Free Expansion only:
Forces are constantly being applied between molecules-molecules and molecules-enclosure due to the kinetic energy of the gas molecules.
Everything is in balance and the gas molecules are pushing against all sides of the container equally.
Now the container is enlarged by opening the partition that contains "nothing" (a vacuum).
No work done, as the heat/kinetic energy of the gas/molecules does not change, but the same kinetic energy will still distribute them evenly throughout the enlarged volume (moving into the vacuum implies, there must be a force that is responsible for it to happen).
Simply because there is no resistance from the vacuum and consequently random movement will go on until a balanced state ("equilibrium") is reached.
The forces on the enclosing container and between molecules then balance each other out again. No work done (comparing initial and end state; in between things are "in a flow").
The only difference being, that the pressure is reduced proportionally to the enlarging of the volume.
That would be how Free Expansion works.
If, however, you open the container, that balance is destroyed.
There will still be forces on the enclosure, but the balancing force is missing where the container is open.
(And that is not even taking into account, that in the case of a rocket engine the effect is amplified by the exothermic reaction as it generates heat and additional hot gas.)
=> Gas molecules will "leave" the container without exerting force on it. At the same time, gas molecules will still be "pushing" at the opposite side of the container.
=> Newton's Third Law. The container experiences a force in the opposite direction of the opening, equivalent to the force, that's expelling the gas molecules through the opening (strictly speaking, it's equivalent to the energy the expelled gas molecules take with them instead of counter-balancing the remaining forces).
=> Now the balance of forces is no longer internal to the container, but it is between the container (rocket) going one way and gas molecules going the other way.
This is, why you cannot ignore the effect of the enclosure/container. Expanding into a vacuum within the same container is not the same as "expanding" into any vacuum.
This is why Joule's Law of Free Expansion describes something different than what's happening to create thrust in a rocket.
This is also, why a rocket will work regardless of what is (or isn't) outside.
The balance of forces (active-reactive) is not happening outside.If there is an outer pressure to resist the exhaust flow then we will have an active force and reactive force - Newton's laws.
No outer pressure ,vacuum conditions , then no work done = no active or reactive force . Joules law. Gas (thermal energy) expands freely into the vacuum.
The active-reactive force
cannot be created by outer pressure.
Outer pressure could push a rocket forward, like wind pushes a sailboat forward, but that would be regardless of the sails providing additional against the wind or simply being there.
If the rocket wasn't ignited yet, that outer pressure would still be there, applying force to the back of the rocket ... so why doesn't it move the rocket?
Because there is equal pressure from the environment all around the rocket; the effects of "outer pressure" cancel each out.
As to Joule's Law ... see above.
Did that clarify my statement?
I'll be happy to answer any remaining questions.
iC