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Some questions regarding classical mechanics in space

  1. Sep 20, 2011 #1
    I'm working on a project for a space habitat, and I want to have the math to back it up. Sorry if some of these seem obvious or too simple, but I'm here to learn. By the way this is not homework, it's just something I do in my spare time. And if you have the links where I can learn more about these things, just provide those and I'll learn on my own.

    1. Let's say you have a circular object, which can spin around an axis (a metal rod, for example) in space, all power by an electrical motor (so because of EM). Is it possible that because of the difference in mass and the fact that it's in space, that the rod spins inside the circular object and not the other way around (the circular object spinning around a rod).

    2. I know that air is made of 70% nitrogen, but we only use in the breathing process O2 and the plants only use CO2, so what is the role of the nitrogen. Is it to make the air (the whole compound) lighter so we can breathe it, or what?

    3. How do rockets propel themselves in space? On earth, the blades simply push back the air, but in space there is no air. And can inertia a planet's orbit be used for moving forward (Be on a course forward, engage on a orbit, spin around, the break off the orbit and start moving independently but in a different direction).

    4. Is the temperature in space constant, and if so is it 0 K? (If it's something like 1/1000 of a degree above 0 K it's not that relevant, but interesting nonetheless)
  2. jcsd
  3. Sep 20, 2011 #2


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    Of course. However you must remember that unless you have something actively working to counteract the rotation of the shaft, then the object will still spin. Similar to how a helicoptor will spin opposite of it's main rotor if it didn't have a tail rotor.

    A 100% oxygen atmosphere is VERY dangerous. Apollo 1 caught fire which resulted in the death of all 3 astronauts because they used 100% oxygen at near atmospheric pressure. In contrast Nitrogen is almost completely inert and will help keep a fire from occuring compared to having 100% oxygen. The ISS has an atmosphere that is very close to Earth's.

    Rockets use the combustion of fuel which heats and pressurizes the exhaust and forces it out the back through the nozzle at very high speeds. This pushes the rocket forwards. (and whatever is attached to the rocket) Rocket fuel carries it's own oxidizer (Oxygen or a similar substance) so that the combustion process can take place. One such fuel is Liquid Oxygen mixed with Liquid Hydrogen. This produces a lot of thrust and the resulting exhaust is simply water.

    The concept of a temperature in space is not like most people think. Typically we use temperature to refer to the collective motion of the molecules and atoms that make up a substance, such as air or water. The faster the motion of all these particles, the higher the temperature. Since space is composed of a near vacuum, it's temperature is much less relevant. While the heat of a few particles in a cubic meter of space might be high or low, heat transfer to or from the spacecraft is extemely low. Without something like air or water to absorb the heat, a spacecraft can only radiate the heat away as thermal radiation. (What you feel when sitting near a campfire) One of the design features of most spacecraft and satellites is shielding to either store heat or get rid of it. When in direct sunlight the spacecraft can heat up to very high temperatures and when in shadow it can cool to very low temps, both of which are bad for it and are the reason for such shielding.

    To answer your question, the temperature in space varies by a very large degree.
  4. Sep 20, 2011 #3
    To the first question I would hate to see the size of motor that will turn a habitat, Its far easier to use thrusters to cause your artificial gravity needs, Due to lack of friction it would only require as much energy as its mass dictates by netwonian laws. The shaft itself would spin as you would have to mount that motor to some form of structure that structure would have to have more mass thus requiring more energy to cause centrifical motion than the habitat mounted to the shaft. That cetrifical motion would have to be sufficient to create a centrifical force equal to gravity at sea level otherwise human bone density gets softer.

    The next question concerning breathable O2 levels I suggest you look at dive tanks for your answer breathing straight O2 is harmful if I recall correctly your best off with a mixture set to the pressure, I won't get to far into this as I'm only passingly familiar with the pro's and cons of mixtures. The main key here is pressure balancing to avoid effects such as the bends. Divers if I recall have different mixtures adjusted to handle certain depths.

    The average temperature of space is around 3 k and is an average temperature different points in space could be colder or warmer, however that is only a small problem, A larger concern is the cosmic radiation in space once you get outside Earths protective magnetosphere.

    Rocket fuel has its own oxygen to support combustion, Space is never truly empty just a lot less dense, so rocket fuel does have matter to push against though this isn't enough for propelers to function as propellers work mainly by creating a high pressure zone on one side and a low pressure zone on the other.

    The other problem is flying debris which moves at extremely high rates even the smallest dust particle can cause significant damage at those speeds.
    I good way to learn some of the problems is to look at what NASA went through in its space station design.

    Drakith was typing the same time as I was lol I like many of his answers better
  5. Sep 22, 2011 #4
    I'm sorry but this is just wrong. The space is pretty empty and what little stuff there is is absolutely not needed for rocket to push against. The rocket carries the stuff to "push against" with it, this is the whole point of a rocket. In fact any ambient pressure reduces the effectiveness of a rocket engine because it limits the expansion rate of the nozzle (not to mention the aerodynamic resistance).
  6. Sep 22, 2011 #5
    Yeah after reading it I realized I misrepresented it. Space does have matter but your correct in that its not needed to generate thrust the way forces act in equal but opposite forces is sufficient on its own. Even empty space has fields in it. The average density is 1 atom per cm^3.

    It is often a misconception that space is a vacuum or simply empty. Space is a nearly perfect vacuum, even better than the best ones made in labs on earth, but it is not devoid of everything. The fact is that space is filled with tiny particles called cosmic dust and elements like hydrogen and helium. This applies for interstellar space also and all the previously mention particles make up what is known as the interstellar medium.
    Last edited: Sep 22, 2011
  7. Sep 22, 2011 #6
    A rocket does not need air to "push against". The fuel inside the rocket combusts, expands, and exits the rocket engine at a fast rate of speed. For every action there's an equal yet opposite reaction!
  8. Sep 22, 2011 #7
    I didn't read all the other responses so sorry if they have been answered already. Here's the deal:

    1. Both will spin such that the total angular momentum is the same as when you started. The rod will spin around your "object" and your "object" will spin around the rod. The one with larger mass will spin slower.

    2. Nitrogen is there to increase the total pressure. If you put in 90% oxygen, your lungs actually don't benefit too much from it (I remember doing a problem like this in stat. mech class). 21% oxygen is actually perfect for our lungs, increasing it barely has any effect. I'm sure there are other biological effects that I don't know about too.

    3. Rockets always propel themselves in the same manner: shooting mass out the back. Doesn't matter if there is air or not. In fact, it's better if there is no air.

    4. Temperature cannot be defined in empty "space". Temperature is a statistical measure of a collection of particles; for example, in the microcanonical ensemble it is a measure of the average speed of the particles (if you have particles of 1 kind).
    But if you leave an object in space, it will radiate until it reaches equilibrium with "background radiation" which is very low. An object can get very cold this way because photons are given a lot of space to go, and there is a lot of entropy to be made that way.
  9. Sep 22, 2011 #8


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    Just to elaborate this Curl, an object near a star will be at a much higher temp than the "background" radiation. (By background radiation I take that as the CMB) In space near the sun an object will be at a much higher temperature than it would be if it were halfway between Alpha Centauri and the Sun. Also, what do you mean by "photons are given a lot of space to go"?
  10. Sep 22, 2011 #9
    It is more or less an infinite "box" they are confined to, unlike the finite box examples you'll see in many books. Thus, the density of photons per unit volume is very low.
  11. Sep 22, 2011 #10


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    Are you saying that in an average area of space, the intensity of EM radiation is very low, causing objects to radiate their heat away until they reach equilibrium with the amount of "background" radiation at a very low temperature?
  12. Sep 23, 2011 #11
    not area, but volume.
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