by bongostaple on March 29th, 2018, 3:25 pm.
Your dismissal of air or ground making a total of three forces and not a pair, is fundamentally wrong. Anything fired out the exhaust of a rocket will be met with resistance from any kind of matter that's in that location, whether it's solid or fluid. . . . whatever the rocket emits is effectively pushing against what's just outside, and all matter takes some effort to move. Therefore your three things are just two things.
Certainly air and ground possess the ability to resist other bodies. I dismiss air or ground only as participating in the paired objects of rocket & exhaust and the forces emanating from them. All forces of material bodies are paired. Material bodies are incapable of emitting an unpaired force.
The fact that forces are inevitably paired is important to rocketry as to everything else: The rocket's rise, a force vector, must be paired with the only other one available-- the exhaust's departure.
"Therefore your three things are just two things."
I'm not sure I understand you. If you mean something different, please say what. I think you are saying that the rocket's exhaust is pushing against either ground or air, and this is Newton's 3d, this is the pair of opposing forces which is lifting the rocket.
Back to my wheeled chair; this time I have 14 sandbags on my lap. I throw one, it hits the ground (downward vector) & the ground resists (upward vector). The 2 force vectors have cancelled each other (just as if you were pushing on a wall), but let's ignore that. In my understanding of what you mean there is no leftover vector representing the rocket's ascent, but ignore that.
I continue throwing sandbags and each departure from my hands causes my chair to move back by the same amount. Why aren't the earlier-thrown sandbags also causing my chair to move? Because only departures between paired objects and their forces will cause motion. Obvious in the chair-sandbag eg, of course.
You will think that it's different with exhaust because it looks continuous. Each layer of exhaust molecules/particles as it departs at great speed is paired with an opposite propulsive force on the rocket. It doesn't matter whether all the energy comes from the gas due to heating and compression, or whether it also comes from a push-piston or whatever in the rocket.
Do you still think that the continuous plume of exhaust is pushing off the ground at the far end and since it's in contact with the rocket at the near end it's pushing the rocket up? You have seen powerful exhaust come out & seen lift-off. I am asking you to ignore this perceptual conjunction, and instead apply Newton's 3d which is excrutiatingly nonintuitive.
Here, work with vectors. Represent each layer of departing exhaust with a downward pointing arrow. Say there are 10,000 layers before we have a continuous plume ground-to-rocket. So 10,000 imaginary arrows pointing towards the ground. I say that the earth is resisting the force of the exhaust and the two forces cancel to no motion, like you pushing on the wall.
You (or at least others) are saying that the exhaust force somehow rebounds from the ground, travels up the plume and pushes on the rocket. But if you are designating forces with vectors you can see that this is impossible: The plume is full of downward-pointing arrows. Each & every molecule/particle is moving downwards, and you cannot push your "rebound particles" upward to push on the rocket.
Pardon my not commenting on your evidence that rocketry is a hoax or impossible. I am making only one single point: That the following argument is invalid: Since rocketry relies on exhaust pushing on ground or air, it's impossible in the vacuum of space.
It's my intention to demonstrate only this single point, for reasons entirely friendly to the forum. I am not saying rocketry is possible or is being carried out. It can help no one's credibility to cling to an invalid argument.