4 ms·

I have a similar question. How can a system be truly closed ? It’s always interacting with the stars and galaxies through their gravitational fields. Similarly

• _trampeltier 6y ago
  Also heat, they talk about 1kW energy. Some heat has to go away, or the whole thing would glow and melt soon.
• samus 6y ago
  It can often be considered closed for the purposes of the phenomenon that we want to describe. To be specific, masses like galaxies, stars and planets accelerate all parts of our tiny rocket the same way. Yes, there are differences, but they are usually so small that they can be neglected. Our rocket works mostly the same no matter how it is oriented to these sources of gravity. Of course, as it gets closer to such masses, these forces (called tidal forces) start to matter and will eventually rip it apart, given that it doesn't burn up or collide first. Rockets, differently to balloons, do not depend on an interaction with a gravitic field for their operation. They work by ejecting reaction mass opposite to the flight direction and relying on Newton's third law of motion. Most of the time, the energy to eject the reaction mass is generated by combustion, but it doesn't really matter. Rockets can also be driven by the reaction mass's pressure inside its tank. In general, any system can be turned into a closed system by including all sources of force in the description. Of course, the more forces are included in our model, the less useful it gets. Therefore, physicists and engineers must decide which forces can be ignored. Else, even experiments would become useless because without a model, the experience gained from experiments cannot be generalized.
• PeterisP 6y ago
  As to your first question, distant gravitational fields can be ignored as per theory of relativity - if you're looking at an experiment in a black box (e.g. in an EmDrive) then an otherwise closed system in freefall in gravitational field is absolutely indistinguishable from a system at rest without that field, any experiment would give exactly the same results. And for the second question, mass is energy and energy is mass (as per relativity). Adding or removing energy from a system makes it respectively heavier or lighter; if you heat up an object so that afterwards it has exactly the same atoms but just more internal energy, then it will be slightly heavier than when it was cold - the difference is very small (m=E/c^2, and c^2 is large) but nonzero. If you're looking from outside at the behavior of an isolated black box, it's impossible to detect if inside it some mass is "lost" in a nuclear reaction i.e. converted to energy, as both the inertial and gravitational mass of a system is equivalent to the sum of both rest mass and energy, and that total is conserved. If that box pushes out energy/mass to outside or gains energy/mass from outside, then of course it is not a closed system. But converting rest mass to energy and vice versa in a chemical or nuclear reaction would not make it a non-closed system as long as all the inputs and outputs stay in the system, as the total gravitational effect of the system before and after the reaction would be exactly the same.