by patrix » April 2nd, 2018, 7:31 pm
So the NASA rockets move by pushing at themselves with "superheated" compressed gas from the inside then. Same old same old. Physics says no
If the movement of the gas inside the rocket causes it to exit, that movement will propel the rocket.
If the movement of the gas inside the rocket is prevented from causing exit, the rocket will not be propelled. (provided the gas has enough mass & velocity, of course)
Try it yourself. Sit in a wheeled chair on a smooth surface with a 40 lb dog on your lap. You, the dog & chair are all considered one object now, one mass (like the gas inside the rocket is included in the rocket's mass). The dog may make all the attempts he likes to jump free, but so long as you hold onto him, you are still considered as one mass. But if you open your arms so that he actually jumps free, your chair will be propelled backward. It doesn't matter whether he jumps into air or vacuum. Only his movements and velocity while he is still on the way to exit matter. When his paws are clear of you he can no longer affect the chair's movement.
If no dog, use sand in a garbage bag.
The idea that the rocket is propelled by its exhaust pushing on the air is a misunderstanding of Newton's 3d, which deals with paired objects and the forces they exert. Rocket and gases until separation are a pair involved in the transactional force which gives both a motion which separates them.
The gases after separation are exhaust. Within atmosphere the air resists the exhaust. The exhaust being initially more energetic pushes it aside, continuing to travel away from the rocket until equlibrium is reached. This going-away exhaust does not affect the rocket, even if it goes into vacuum.
Even though when he made those exact same movements while you prevented his leap your chair did not move.