Patrix, it's my understanding that the very heart of your argument is that "Newton's 3d is not valid in space (vacuum). I agree with you that your version of Newton's 3d isn't valid in space.Postby patrix on March 28th, 2018, 7:09 am
Oh please Penelope I am not "ignoring/invalidating" Newtons 3rd. Or "his" 1 or 2 either.
I'm making the point that your version of Newton's 3d doesn't work here on Earth either, because it is a mistaken interpretation of Newton's 3d.
I can't think that discussing the effects of a mistaken interpretation of the Free Expansion experiment upon a mistaken interpretation of Newton's 3d will possibly illuminate the matter. I challenge you to defend your version of Newton's 3d here on earth where we are aided by observation and have the hope of some intuitive knowledge or perception.Patrix said, I'm explaining that they never come to play when gas expands in vacuum since no action/reaction occurs.
Let me show you why I think your version of Newton's 3d is false.
As you know, Newton's 3d states that when material objects interact the force that object A exerts upon Object B must be synchronous and equal in magnitude to an obligatory force which Object B exerts on Object A, but in a direction 180 degrees opposite. "Obligatory" because there are no unmatched forces, there are only paired forces.
STANDARD NEWTON'S 3D AT LIFT-OFF
At lift-off the force vector which indicates the rise of the rocket is a line from bottom to top of the rocket with an arrowhead at the top. This vector MUST be matched by one in the opposite direction, so we draw another in the opposite direction indicating en masse the many exiting molecules and particles of the exhaust. The exhaust arrowhead points downward 180 degrees opposite to the rise of the rocket. Newton's 3d is satisfied and, provided the mass x velocity of the exhaust is sufficient, the rocket rises.
YOUR VERSION OF NEWTON'S 3D AT LIFT-OFF
Let's start at the element that you have added to Newton's 3d, the "push off" of the exhaust gases upon the ground as a means of raising the rocket: The exhaust vector points downward and is met by one pointing upward from the ground, indicating that the ground is exerting a resistant force equal in magnitude to that of the exhaust vector. Now what? We have stasis, cancellation of forces. How do we get from this a vector to push on the rocket? (Not even a supplementary force.) You have paired the forces of your entities in such a way that your rocket will never rise.
I say "you", Patrix, but I mean anyone's adherence to the idea that a rocket can lift off only if its exhaust pushes on the ground; or propel itself only by pushing on air. It's just that you've been the only one brave enough to speak up. And I do appreciate your response.
I don't mean to be unresponsive to your point about Free Expansion, so I will say this much: The reason for Boetius' inserting it into the discussion was to give scientific buttress to the idea that rockets cannot propel themselves by trying to gain traction through spewing exhaust against vacuum.
But Patrix, this is not an idea at issue. I agree. I'll wager so would FervidGus. No one doubts that pushing on vacuum will get you nowhere; in fact it is impossible to push on vacuum, for it doesn't resist and you therefore have a violation of Newton's 3d regarding paired forces. You try, right now, to push on air & you'll see what I mean.
Rockets don't use this method of propulsion; they have Newton's 3d. The real one, sans pushing their exhaust against ground, air or vacuum.